Aperiodic tracking reference signals for sidelink communications

ABSTRACT

Wireless communications systems and methods related to communicating information are provided. A method of wireless communication performed by a first sidelink user equipment (UE) may include receiving, from a second sidelink UE, a tracking reference signal (TRS), synchronizing time and frequency with the second sidelink UE based on the TRS, and receiving a physical sidelink shared channel (PSSCH) communication based on the time and frequency synchronization with the second sidelink UE.

TECHNICAL FIELD

This application relates to wireless communication systems, and more particularly, to configuring tracking reference signals for sidelink communications.

INTRODUCTION

Wireless communications systems are widely deployed to provide various types of communication content such as voice, video, packet data, messaging, broadcast, and so on. These systems may be capable of supporting communication with multiple users by sharing the available system resources (e.g., time, frequency, and power). A wireless multiple-access communications system may include a number of base stations (BSs), each simultaneously supporting communications for multiple communication devices, which may be otherwise known as user equipment (UE).

To meet the growing demands for expanded mobile broadband connectivity, wireless communication technologies are advancing from the LTE technology to a next generation new radio (NR) technology. For example, NR is designed to provide a lower latency, a higher bandwidth or throughput, and a higher reliability than LTE. NR is designed to operate over a wide array of spectrum bands, for example, from low-frequency bands below about 1 gigahertz (GHz) and mid-frequency bands from about 1 GHz to about 6 GHz, to high-frequency bands such as millimeter wave (mmWave) bands. NR is also designed to operate across different spectrum types, from licensed spectrum to unlicensed and shared spectrum. Spectrum sharing enables operators to opportunistically aggregate spectrums to dynamically support high-bandwidth services. Spectrum sharing can extend the benefit of NR technologies to operating entities that may not have access to a licensed spectrum.

NR may support various deployment scenarios to benefit from the various spectrums in different frequency ranges, licensed and/or unlicensed, and/or coexistence of the LTE and NR technologies. For example, NR can be deployed in a standalone NR mode over a licensed and/or an unlicensed band or in a dual connectivity mode with various combinations of NR and LTE over licensed and/or unlicensed bands.

In a wireless communication network, a BS may communicate with a UE in an uplink direction and a downlink direction. Sidelink was introduced in LTE to allow a UE to send data to another UE (e.g., from one vehicle to another vehicle) without tunneling through the BS and/or an associated core network. The LTE sidelink technology has been extended to provision for device-to-device (D2D) communications, vehicle-to-everything (V2X) communications, and/or cellular vehicle-to-everything (C-V2X) communications. Similarly, NR may be extended to support sidelink communications, D2D communications, V2X communications, and/or C-V2X over licensed frequency bands and/or unlicensed frequency bands (e.g., shared frequency bands).

BRIEF SUMMARY OF SOME EXAMPLES

The following summarizes some aspects of the present disclosure to provide a basic understanding of the discussed technology. This summary is not an extensive overview of all contemplated features of the disclosure and is intended neither to identify key or critical elements of all aspects of the disclosure nor to delineate the scope of any or all aspects of the disclosure. Its sole purpose is to present some concepts of one or more aspects of the disclosure in summary form as a prelude to the more detailed description that is presented later.

In an aspect of the disclosure, a method of wireless communication performed by a first sidelink user equipment (UE) may include receiving, from a second sidelink UE, a tracking reference signal (TRS), synchronizing time and frequency with the second sidelink UE based on the TRS, and receiving a physical sidelink shared channel (PSSCH) communication based on the time and frequency synchronization with the second sidelink UE.

In an additional aspect of the disclosure, a method of wireless communication performed by a first sidelink user equipment (UE) may include transmitting, to a second sidelink UE, a tracking reference signal (TRS), wherein the TRS is configured for the second sidelink UE to synchronize time and frequency with the first sidelink UE based on the TRS, and transmitting, to the second sidelink UE synchronized in time and frequency with the first sidelink UE, a sidelink communication.

In an additional aspect of the disclosure, a first sidelink user equipment (UE) may include a memory; a transceiver; and at least one processor coupled to the memory and the transceiver, wherein the first sidelink UE is configured to receive, from a second sidelink UE, a tracking reference signal (TRS), synchronize time and frequency with the second sidelink UE based on the TRS, and receive a physical sidelink shared channel (PSSCH) communication based on the time and frequency synchronization with the second sidelink UE.

In an additional aspect of the disclosure, a first sidelink user equipment (UE) may include a memory; a transceiver; and at least one processor coupled to the memory and the transceiver, wherein the first sidelink UE is configured to transmit, to a second sidelink UE, a tracking reference signal (TRS), wherein the TRS is configured for the second sidelink UE to synchronize time and frequency with the first sidelink UE based on the TRS and transmit, to the second sidelink UE synchronized in time and frequency with the first sidelink UE, a sidelink communication.

Other aspects, features, and instances of the present invention will become apparent to those of ordinary skill in the art, upon reviewing the following description of specific, exemplary instances of the present invention in conjunction with the accompanying figures. While features of the present invention may be discussed relative to certain aspects and figures below, all instances of the present invention can include one or more of the advantageous features discussed herein. In other words, while one or more instances may be discussed as having certain advantageous features, one or more of such features may also be used in accordance with the various instances of the invention discussed herein. In similar fashion, while exemplary aspects may be discussed below as device, system, or method instances it should be understood that such exemplary instances can be implemented in various devices, systems, and methods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a wireless communication network according to some aspects of the present disclosure.

FIG. 2 illustrates a sidelink wireless communication network according to some aspects of the present disclosure.

FIGS. 3-4 illustrate tracking reference signal resources according to some aspects of the present disclosure.

FIG. 5 illustrates sidelink UE groups according to some aspects of the present disclosure.

FIGS. 6A and 6B are signaling diagrams of a wireless communication method according to some aspects of the present disclosure.

FIGS. 7A and 7B are signaling diagrams of a wireless communication method according to some aspects of the present disclosure.

FIG. 8 is a block diagram of an exemplary user equipment (UE) according to some aspects of the present disclosure.

FIG. 9 is a block diagram of an exemplary base station (BS) according to some aspects of the present disclosure.

FIG. 10 is a flow diagram of a communication method according to some aspects of the present disclosure.

FIG. 11 is a flow diagram of a communication method according to some aspects of the present disclosure.

DETAILED DESCRIPTION

The detailed description set forth below, in connection with the appended drawings, is intended as a description of various configurations and is not intended to represent the only configurations in which the concepts described herein may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of the various concepts. However, it will be apparent to those skilled in the art that these concepts may be practiced without these specific details. In some instances, well-known structures and components are shown in block diagram form in order to avoid obscuring such concepts.

This disclosure relates generally to wireless communications systems, also referred to as wireless communications networks. In various instances, the techniques and apparatus may be used for wireless communication networks such as code division multiple access (CDMA) networks, time division multiple access (TDMA) networks, frequency division multiple access (FDMA) networks, orthogonal FDMA (OFDMA) networks, single-carrier FDMA (SC-FDMA) networks, LTE networks, GSM networks, 5^(th) Generation (5G) or new radio (NR) networks, as well as other communications networks. As described herein, the terms “networks” and “systems” may be used interchangeably.

An OFDMA network may implement a radio technology such as evolved UTRA (E-UTRA), Institute of Electrical and Electronic Engineers (IEEE) 802.11, IEEE 802.16, IEEE 802.20, flash-OFDM and the like. UTRA, E-UTRA, and Global System for Mobile Communications (GSM) are part of universal mobile telecommunication system (UMTS). In particular, long term evolution (LTE) is a release of UMTS that uses E-UTRA. UTRA, E-UTRA, GSM, UMTS and LTE are described in documents provided from an organization named “3rd Generation Partnership Project” (3GPP), and cdma2000 is described in documents from an organization named “3rd Generation Partnership Project 2” (3GPP2). These various radio technologies and standards are known or are being developed. For example, the 3rd Generation Partnership Project (3GPP) is a collaboration between groups of telecommunications associations that aims to define a globally applicable third generation (3G) mobile phone specification. 3GPP long term evolution (LTE) is a 3GPP project which was aimed at improving the universal mobile telecommunications system (UMTS) mobile phone standard. The 3GPP may define specifications for the next generation of mobile networks, mobile systems, and mobile devices. The present disclosure is concerned with the evolution of wireless technologies from LTE, 4G, 5G, NR, and beyond with shared access to wireless spectrum between networks using a collection of new and different radio access technologies or radio air interfaces.

In particular, 5G networks contemplate diverse deployments, diverse spectrum, and diverse services and devices that may be implemented using an OFDM-based unified, air interface. In order to achieve these goals, further enhancements to LTE and LTE-A are considered in addition to development of the new radio technology for 5G NR networks. The 5G NR will be capable of scaling to provide coverage (1) to a massive Internet of things (IoTs) with an ultra-high density (e.g., ˜1M nodes/km²), ultra-low complexity (e.g., ˜10 s of bits/sec), ultra-low energy (e.g., ˜10+ years of battery life), and deep coverage with the capability to reach challenging locations; (2) including mission-critical control with strong security to safeguard sensitive personal, financial, or classified information, ultra-high reliability (e.g., ˜99.9999% reliability), ultra-low latency (e.g., ˜1 ms), and users with wide ranges of mobility or lack thereof; and (3) with enhanced mobile broadband including extreme high capacity (e.g., ˜10 Tbps/km²), extreme data rates (e.g., multi-Gbps rate, 100+ Mbps user experienced rates), and deep awareness with advanced discovery and optimizations.

The 5G NR may be implemented to use optimized OFDM-based waveforms with scalable numerology and transmission time interval (TTI); having a common, flexible framework to efficiently multiplex services and features with a dynamic, low-latency time division duplex (TDD)/frequency division duplex (FDD) design; and with advanced wireless technologies, such as massive multiple input, multiple output (MIMO), robust millimeter wave (mmWave) transmissions, advanced channel coding, and device-centric mobility. Scalability of the numerology in 5G NR, with scaling of subcarrier spacing, may efficiently address operating diverse services across diverse spectrum and diverse deployments. For example, in various outdoor and macro coverage deployments of less than 3 GHz FDD/TDD implementations, subcarrier spacing may occur with 15 kHz, for example over 5, 10, 20 MHz, and the like bandwidth (BW). For other various outdoor and small cell coverage deployments of TDD greater than 3 GHz, subcarrier spacing may occur with 30 kHz over 80/100 MHz BW. For other various indoor wideband implementations, using a TDD over the unlicensed portion of the 5 GHz band, the subcarrier spacing may occur with 60 kHz over a 160 MHz BW. Finally, for various deployments transmitting with mmWave components at a TDD of 28 GHz, subcarrier spacing may occur with 120 kHz over a 500 MHz BW.

The scalable numerology of the 5G NR facilitates scalable TTI for diverse latency and quality of service (QoS) requirements. For example, shorter TTI may be used for low latency and high reliability, while longer TTI may be used for higher spectral efficiency. The efficient multiplexing of long and short TTIs to allow transmissions to start on symbol boundaries. 5G NR also contemplates a self-contained integrated subframe design with uplink/downlink scheduling information, data, and acknowledgement in the same subframe. The self-contained integrated subframe supports communications in unlicensed or contention-based shared spectrum, adaptive uplink/downlink that may be flexibly configured on a per-cell basis to dynamically switch between uplink and downlink to meet the current traffic needs.

Various other aspects and features of the disclosure are further described below. It should be apparent that the teachings herein may be embodied in a wide variety of forms and that any specific structure, function, or both being disclosed herein is merely representative and not limiting. Based on the teachings herein one of an ordinary level of skill in the art should appreciate that an aspect disclosed herein may be implemented independently of any other aspects and that two or more of these aspects may be combined in various ways. For example, an apparatus may be implemented or a method may be practiced using any number of the aspects set forth herein. In addition, such an apparatus may be implemented or such a method may be practiced using other structure, functionality, or structure and functionality in addition to or other than one or more of the aspects set forth herein. For example, a method may be implemented as part of a system, device, apparatus, and/or as instructions stored on a computer readable medium for execution on a processor or computer. Furthermore, an aspect may comprise at least one element of a claim.

The deployment of NR over an unlicensed spectrum is referred to as NR-unlicensed (NR-U). Federal Communications Commission (FCC) and European Telecommunications Standards Institute (ETSI) are working on regulating 6 GHz as a new unlicensed band for wireless communications. The addition of 6 GHz bands allows for hundreds of megahertz (MHz) of bandwidth (BW) available for unlicensed band communications. Additionally, NR-U can also be deployed over 2.4 GHz unlicensed bands, which are currently shared by various radio access technologies (RATs), such as IEEE 802.11 wireless local area network (WLAN) or WiFi and/or license assisted access (LAA). Sidelink communications may benefit from utilizing the additional bandwidth available in an unlicensed spectrum. However, channel access in a certain unlicensed spectrum may be regulated by authorities. For instance, some unlicensed bands may impose restrictions on the power spectral density (PSD) and/or minimum occupied channel bandwidth (OCB) for transmissions in the unlicensed bands. For example, the unlicensed national information infrastructure (UNII) radio band has a minimum OCB requirement of about 70 percent (%).

Some sidelink systems may operate over a 20 MHz bandwidth in an unlicensed band. A BS may configure a sidelink resource pool over the 20 MHz band for sidelink communications. A sidelink resource pool is typically partitioned into multiple frequency subchannels or frequency subbands (e.g., about 5 MHz each) and a sidelink UE may select a sidelink resource (e.g., a subchannel) from the sidelink resource pool for sidelink communication. To satisfy an OCB of about 70%, a sidelink resource pool may utilize a frequency-interlaced structure. For instance, a frequency-interlaced-based sidelink resource pools may include a plurality of frequency interlaces over the 20 MHz band, where each frequency interlace may include a plurality of resource blocks (RBs) distributed over the 20 MHz band. For example, the plurality of RBs of a frequency interlace may be spaced apart from each other by one or more other RBs in the 20 MHz unlicensed band. A sidelink UE may select a sidelink resource in the form of frequency interlaces from the sidelink resource pool for sidelink communication. In other words, sidelink transmissions may utilize a frequency-interlaced waveform to satisfy an OCB of the unlicensed band. However, S-SSBs may be transmitted in a set of contiguous RBs, for example, in about eleven contiguous RBs. As such, S-SSB transmissions alone may not meet the OCB requirement of the unlicensed band. Accordingly, it may be desirable for a sidelink sync UE to multiplex an S-SSB transmission with one or more channel state information reference signals (CSI-RSs) in a slot configured for S-SSB transmission so that the sidelink sync UE's transmission in the slot may comply with an OCB requirement.

The present application describes mechanisms for a sidelink UE to multiplex an S-SSB transmission with a CSI-RS transmission in a frequency band to satisfy an OCB of the frequency band. For instance, the sidelink UE may determine a multiplex configuration for multiplexing a CSI-RS transmission with an S-SSB transmission in a sidelink BWP. The sidelink UE may transmit the S-SSB transmission in the sidelink BWP during a sidelink slot. The sidelink UE may transmit one or more CSI-RSs in the sidelink BWP during the sidelink slot by multiplexing the CSI-RS and the S-SSB transmission based on the multiplex configuration.

In some aspects, the sidelink UE may transmit the S-SSB transmission at an offset from a lowest frequency of the sidelink BWP based on a synchronization raster (e.g., an NR-U sync raster). In some aspects, the sidelink UE may transmit the S-SSB transmission aligned to a lowest frequency of the sidelink BWP. For instance, a sync raster can be defined for sidelink such that the S-SSB transmission may be aligned to a lowest frequency of the sidelink BWP.

In some aspects, the multiplex configuration includes a configuration for multiplexing the S-SSB transmission with a frequency interlaced waveform sidelink transmission to meet the OCB requirement. For instance, the sidelink transmission may include a CSI-RS transmission multiplexed in frequency within a frequency interlace with RBs spaced apart in the sidelink BWP. In some instances, the sidelink UE may rate-match the CSI-RS transmission around RBs that are at least partially overlapping with the S-SSB transmission.

In some aspects, the multiplex configuration includes a configuration for multiplexing the S-SSB transmission with a subchannel-based sidelink transmission to meet the OCB requirement. For instance, the sidelink transmission may include a CSI-RS transmission multiplexed in time within a subchannel including contiguous RBs in the sidelink BWP. For instance, the S-SSB transmission may be transmitted at a low frequency portion of the sidelink BWP, and the CSI-RS may be transmitted in a subchannel located at a high frequency portion of the sidelink BWP to meet the OCB.

In some aspects, a BS may configure different sidelink resource pools for slots that are associated with S-SSB transmissions and for slots that are not associated with S-SSB transmissions. For instance, the BS may configure a first resource pool with a frequency-interlaced structure for slots that are not configured for S-SSB transmissions. The first resource pool may include a plurality of frequency interlaces (e.g., distributed RBs), where each frequency interlace may carry a PSCCH/PSSCH transmission. The BS may configure a second resource pool with a subchannel-based structure for slots that are configured for S-SSB transmission. The second resource pool may include a plurality of frequency subchannels (e.g., contiguous RBs), where each subchannel may carry a PSCCH/PSSCH transmission. To satisfy an OCB in a sidelink slot configured for an S-SSB transmission, the sidelink UE (e.g., a sidelink sync UE) may transmit an S-SSB transmission multiplexed with a CSI-RS transmission. For instance, the S-SSB transmission may be transmitted in frequency resources located at a lower frequency portion of a sidelink BWP and the CSI-RS transmission may be transmitted in frequency resources located at higher frequency portion of the sidelink BWP.

Aspects of the present disclosure may provide several benefits. For example, providing additional opportunities for a sidelink sync UE to transmit S-SSBs may increase the number of UEs synchronized in the wireless network. Maintaining synchronization among the UEs in the sidelink wireless network may increase the reliability and throughput of the network.

FIG. 1 illustrates a wireless communication network 100 according to some aspects of the present disclosure. The network 100 includes a number of base stations (BSs) 105 and other network entities. A BS 105 may be a station that communicates with UEs 115 and may also be referred to as an evolved node B (eNB), a next generation eNB (gNB), an access point, and the like. Each BS 105 may provide communication coverage for a particular geographic area. In 3GPP, the term “cell” can refer to this particular geographic coverage area of a BS 105 and/or a BS subsystem serving the coverage area, depending on the context in which the term is used.

A BS 105 may provide communication coverage for a macro cell or a small cell, such as a pico cell or a femto cell, and/or other types of cell. A macro cell generally covers a relatively large geographic area (e.g., several kilometers in radius) and may allow unrestricted access by UEs with service subscriptions with the network provider. A small cell, such as a pico cell, would generally cover a relatively smaller geographic area and may allow unrestricted access by UEs with service subscriptions with the network provider. A small cell, such as a femto cell, would also generally cover a relatively small geographic area (e.g., a home) and, in addition to unrestricted access, may also provide restricted access by UEs having an association with the femto cell (e.g., UEs in a closed subscriber group (CSG), UEs for users in the home, and the like). A BS for a macro cell may be referred to as a macro BS. A BS for a small cell may be referred to as a small cell BS, a pico BS, a femto BS or a home BS. In the example shown in FIG. 1 , the BSs 105 d and 105 e may be regular macro BSs, while the BSs 105 a-105 c may be macro BSs enabled with one of three dimension (3D), full dimension (FD), or massive MIMO. The BSs 105 a-105 c may take advantage of their higher dimension MIMO capabilities to exploit 3D beamforming in both elevation and azimuth beamforming to increase coverage and capacity. The BS 105 f may be a small cell BS which may be a home node or portable access point. A BS 105 may support one or multiple (e.g., two, three, four, and the like) cells.

The network 100 may support synchronous or asynchronous operation. For synchronous operation, the BSs may have similar frame timing, and transmissions from different BSs may be approximately aligned in time. For asynchronous operation, the BSs may have different frame timing, and transmissions from different BSs may not be aligned in time.

The UEs 115 are dispersed throughout the wireless network 100, and each UE 115 may be stationary or mobile. A UE 115 may also be referred to as a terminal, a mobile station, a subscriber unit, a station, or the like. A UE 115 may be a cellular phone, a personal digital assistant (PDA), a wireless modem, a wireless communication device, a handheld device, a tablet computer, a laptop computer, a cordless phone, a wireless local loop (WLL) station, or the like. In one aspect, a UE 115 may be a device that includes a Universal Integrated Circuit Card (UICC). In another aspect, a UE may be a device that does not include a UICC. In some aspects, the UEs 115 that do not include UICCs may also be referred to as IoT devices or internet of everything (IoE) devices. The UEs 115 a-115 d are examples of mobile smart phone-type devices accessing network 100. A UE 115 may also be a machine specifically configured for connected communication, including machine type communication (MTC), enhanced MTC (eMTC), narrowband IoT (NB-IoT) and the like. The UEs 115 e-115 h are examples of various machines configured for communication that access the network 100. The UEs 115 i-115 k are examples of vehicles equipped with wireless communication devices configured for communication that access the network 100. A UE 115 may be able to communicate with any type of the BSs, whether macro BS, small cell, or the like. In FIG. 1 , a lightning bolt (e.g., communication links) indicates wireless transmissions between a UE 115 and a serving BS 105, which is a BS designated to serve the UE 115 on the downlink (DL) and/or uplink (UL), desired transmission between BSs 105, backhaul transmissions between BSs, or sidelink transmissions between UEs 115.

In operation, the BSs 105 a-105 c may serve the UEs 115 a and 115 b using 3D beamforming and coordinated spatial techniques, such as coordinated multipoint (CoMP) or multi-connectivity. The macro BS 105 d may perform backhaul communications with the BSs 105 a-105 c, as well as small cell, the BS 105 f. The macro BS 105 d may also transmits multicast services which are subscribed to and received by the UEs 115 c and 115 d. Such multicast services may include mobile television or stream video, or may include other services for providing community information, such as weather emergencies or alerts, such as Amber alerts or gray alerts.

The BSs 105 may also communicate with a core network. The core network may provide user authentication, access authorization, tracking, Internet Protocol (IP) connectivity, and other access, routing, or mobility functions. At least some of the BSs 105 (e.g., which may be an example of an evolved NodeB (eNB) or an access node controller (ANC)) may interface with the core network 130 through backhaul links (e.g., S1, S2, etc.) and may perform radio configuration and scheduling for communication with the UEs 115. In various examples, the BSs 105 may communicate, either directly or indirectly (e.g., through core network), with each other over backhaul links (e.g., X1, X2, etc.), which may be wired or wireless communication links.

The network 100 may also support mission critical communications with ultra-reliable and redundant links for mission critical devices, such as the UE 115 e, which may be a vehicle (e.g., a car, a truck, a bus, an autonomous vehicle, an aircraft, a boat, etc.). Redundant communication links with the UE 115 e may include links from the macro BSs 105 d and 105 e, as well as links from the small cell BS 105 f. Other machine type devices, such as the UE 115 f (e.g., a thermometer), the UE 115 g (e.g., smart meter), and UE 115 h (e.g., wearable device) may communicate through the network 100 either directly with BSs, such as the small cell BS 105 f, and the macro BS 105 e, or in multi-hop configurations by communicating with another user device which relays its information to the network, such as the UE 115 f communicating temperature measurement information to the smart meter, the UE 115 g, which is then reported to the network through the small cell BS 105 f. The network 100 may also provide additional network efficiency through dynamic, low-latency TDD/FDD communications, such as vehicle-to-vehicle (V2V), vehicle-to-everything (V2X), cellular-vehicle-to-everything (C-V2X) communications between a UE 115 i, 115 j, or 115 k and other UEs 115, and/or vehicle-to-infrastructure (V2I) communications between a UE 115 i, 115 j, or 115 k and a BS 105.

In some implementations, the network 100 utilizes OFDM-based waveforms for communications. An OFDM-based system may partition the system BW into multiple (K) orthogonal subcarriers, which are also commonly referred to as subcarriers, tones, bins, or the like. Each subcarrier may be modulated with data. In some instances, the subcarrier spacing between adjacent subcarriers may be fixed, and the total number of subcarriers (K) may be dependent on the system BW. The system BW may also be partitioned into subbands. In other instances, the subcarrier spacing and/or the duration of TTIs may be scalable.

In some instances, the BSs 105 can assign or schedule transmission resources (e.g., in the form of time-frequency resource blocks (RB)) for downlink (DL) and uplink (UL) transmissions in the network 100. DL refers to the transmission direction from a BS 105 to a UE 115, whereas UL refers to the transmission direction from a UE 115 to a BS 105. The communication can be in the form of radio frames. A radio frame may be divided into a plurality of subframes, for example, about 10. Each subframe can be divided into slots, for example, about 2. Each slot may be further divided into mini-slots. In a FDD mode, simultaneous UL and DL transmissions may occur in different frequency bands. For example, each subframe includes a UL subframe in a UL frequency band and a DL subframe in a DL frequency band. In a TDD mode, UL and DL transmissions occur at different time periods using the same frequency band. For example, a subset of the subframes (e.g., DL subframes) in a radio frame may be used for DL transmissions and another subset of the subframes (e.g., UL subframes) in the radio frame may be used for UL transmissions.

The DL subframes and the UL subframes can be further divided into several regions. For example, each DL or UL subframe may have pre-defined regions for transmissions of reference signals, control information, and data. Reference signals are predetermined signals that facilitate the communications between the BSs 105 and the UEs 115. For example, a reference signal can have a particular pilot pattern or structure, where pilot tones may span across an operational BW or frequency band, each positioned at a pre-defined time and a pre-defined frequency. For example, a BS 105 may transmit cell specific reference signals (CRSs) and/or channel state information —reference signals (CSI-RSs) to enable a UE 115 to estimate a DL channel. Similarly, a UE 115 may transmit sounding reference signals (SRSs) to enable a BS 105 to estimate a UL channel. Control information may include resource assignments and protocol controls. Data may include protocol data and/or operational data. In some instances, the BSs 105 and the UEs 115 may communicate using self-contained subframes. A self-contained subframe may include a portion for DL communication and a portion for UL communication. A self-contained subframe can be DL-centric or UL-centric. A DL-centric subframe may include a longer duration for DL communication than for UL communication. A UL-centric subframe may include a longer duration for UL communication than for UL communication.

In some instances, the network 100 may be an NR network deployed over a licensed spectrum. The BSs 105 can transmit synchronization signals (e.g., including a primary synchronization signal (PSS) and a secondary synchronization signal (SSS)) in the network 100 to facilitate synchronization. The BSs 105 can broadcast system information associated with the network 100 (e.g., including a master information block (MIB), remaining minimum system information (RMSI), and other system information (OSI)) to facilitate initial network access. In some instances, the BSs 105 may broadcast the PSS, the SSS, and/or the MIB in the form of synchronization signal blocks (SSBs) over a physical broadcast channel (PBCH) and may broadcast the RMSI and/or the OSI over a physical downlink shared channel (PDSCH).

In some instances, a UE 115 attempting to access the network 100 may perform an initial cell search by detecting a PSS from a BS 105. The PSS may enable synchronization of period timing and may indicate a physical layer identity value. The UE 115 may then receive an SSS. The SSS may enable radio frame synchronization, and may provide a cell identity value, which may be combined with the physical layer identity value to identify the cell. The SSS may also enable detection of a duplexing mode and a cyclic prefix length. The PSS and the SSS may be located in a central portion of a carrier or any suitable frequencies within the carrier.

After receiving the PSS and SSS, the UE 115 may receive a MIB. The MIB may include system information for initial network access and scheduling information for RMSI and/or OSI. After decoding the MIB, the UE 115 may receive RMSI and/or OSI. The RMSI and/or OSI may include radio resource control (RRC) information related to random access channel (RACH) procedures, paging, control resource set (CORESET) for physical downlink control channel (PDCCH) monitoring, physical uplink control channel (PUCCH), physical uplink shared channel (PUSCH), power control, SRS, and cell barring.

After obtaining the MIB, the RMSI and/or the OSI, the UE 115 can perform a random access procedure to establish a connection with the BS 105. For the random access procedure, the UE 115 may transmit a random access preamble and the BS 105 may respond with a random access response. Upon receiving the random access response, the UE 115 may transmit a connection request to the BS 105 and the BS 105 may respond with a connection response (e.g., contention resolution message).

After establishing a connection, the UE 115 and the BS 105 can enter a normal operation stage, where operational data may be exchanged. For example, the BS 105 may schedule the UE 115 for UL and/or DL communications. The BS 105 may transmit UL and/or DL scheduling grants to the UE 115 via a PDCCH. The BS 105 may transmit a DL communication signal to the UE 115 via a PDSCH according to a DL scheduling grant. The UE 115 may transmit a UL communication signal to the BS 105 via a PUSCH and/or PUCCH according to a UL scheduling grant.

In some aspects, the UE 115 c may receive a tracking reference signal (TRS) from the UE 115 a. The UE 115 c may synchronize time and/or frequency with the UE 115 a based on the TRS. The UE 115 c may receive a physical sidelink shared channel (PSSCH) communication from the UE 115 a based on the time and/or frequency synchronization with the UE 115 a. Aspects of the present disclosure may increase time and/or frequency synchronization between the UE 115 c and the UE 115 a (e.g., sidelink UEs) thereby increasing the probability of successful decoding of communications by the UE 115 c.

FIG. 2 illustrates sidelink resources associated with a wireless communication network 200 according to some aspects of the present disclosure. The wireless communications network 200 may include a base station 105 a and UEs 115 a, 115 b, and 115 c, which may be examples of a BS 105 and a UE 115 as described with reference to FIG. 1 . Base station 105 a and UEs 115 a and 115 c may communicate within geographic coverage area 110 a and via communication links 205 a and 205 b, respectively. UE 115 c may communicate with UEs 115 a and 115 b via sidelink communication links 210 a and 210 b, respectively. In some examples, UE 115 c may transmit SCI to UEs 115 a and 115 b via the sidelink control resources 220. The SCI may include an indication of resources reserved for retransmissions by UE 115 c (e.g., the reserved resources 225). In some examples, UEs 115 a and 115 b may determine to reuse one or more of the reserved resources 225.

In some aspects, a device in the wireless communication network 200 (e.g., a UE 115, a BS 105, or some other node) may convey SCI to another device (e.g., another UE 115, a BS 105, sidelink device or vehicle-to-everything (V2X) device, or other node). The SCI may be conveyed in one or more stages. The first stage SCI may be carried on the PSCCH while the second stage SCI may be carried on the corresponding PSSCH. For example, UE 115 c may transmit a PSCCH/first stage SCI 235 (e.g., SCI-1) to each sidelink UE 115 in the network (e.g., UEs 115 a and 115 b) via the sidelink communication links 210. The PSCCH/first stage SCI-1 235 may indicate resources that are reserved by UE 115 c for retransmissions (e.g., the SCI-1 may indicate the reserved resources 225 for retransmissions). Each sidelink UE 115 may decode the first stage SCI-1 to determine where the reserved resources 225 are located (e.g., to refrain from using resources that are reserved for another sidelink transmission and/or to reduce resource collision within the wireless communications network 200). Sidelink communication may include a mode 1 operation in which the UEs 115 are in a coverage area of BS 105 a. In mode 1, the UEs 115 may receive a configured grant from the BS 105 a that defines parameters for the UEs 115 to access the channel. Sidelink communication may also include a mode 2 operation in which the UEs 115 operate autonomously from the BS 105 a and perform sensing of the channel to gain access to the channel. In some aspects, during mode 2 sidelink operations, the sidelink UEs 115 may perform channel sensing to locate resources reserved by other sidelink transmissions. The first stage SCI-1 may reduce the need for sensing each channel. For example, the first stage SCI-1 may include an explicit indication such that the UEs 115 may refrain from blindly decoding each channel. The first stage SCI-1 may be transmitted via the sidelink control resources 220. The sidelink control resources 220 may be configured resources (e.g., time resources or frequency resources) transmitted via a PSCCH 235. In some examples, the PSCCH 235 may be configured to occupy a number of physical resource blocks (PRBs) within a selected frequency. The frequency may include a single subchannel 250 (e.g., 10, 12, 15, 20, 25, or some other number of RBs within the subchannel 250). The time duration of the PSCCH 235 may be configured by the BS 105 a (e.g., the PSCCH 235 may span 1, 2, 3, or some other number of symbols 255).

The first stage SCI-1 may include one or more fields to indicate a location of the reserved resources 225. For example, the first stage SCI-1 may include, without limitation, one or more fields to convey a frequency domain resource allocation (FDRA), a time domain resource allocation (TDRA), a resource reservation period 245 (e.g., a period for repeating the SCI transmission and the corresponding reserved resources 225), a modulation and coding scheme (MCS) for a second stage SCI-2 240, a beta offset value for the second stage SCI-2 240, a DMRS port (e.g., one bit indicating a number of data layers), a physical sidelink feedback channel (PSFCH) overhead indicator, a priority, one or more additional reserved bits, or a combination thereof. The beta offset may indicate the coding rate for transmitting the second stage SCI-2 240. The beta offset may indicate an offset to the MCS index. The MCS may be indicated by an index ranging from 0 to 31. For example, if the MCS is set at index 16 indicating a modulation order of 4 and a coding rate of 378, the beta offset may indicate a value of 2 thereby setting the coding rate to 490 based on an MCS index of 18. In some examples, the FDRA may be a number of bits in the first stage SCI-1 that may indicate a number of slots 238 and a number of subchannels reserved for the reserved resources 225 (e.g., a receiving UE 115 may determine a location of the reserved resources 225 based on the FDRA by using the subchannel 250 including the PSCCH 235 and first stage SCI-1 as a reference). The TDRA may be a number of bits in the first stage SCI-1 (e.g., 5 bits, 9 bits, or some other number of bits) that may indicate a number of time resources reserved for the reserved resources 225. In this regard, the first stage SCI-1 may indicate the reserved resources 225 to the one or more sidelink UEs 115 in the wireless communication network 200.

In some aspects, the BS 105 may configure a sidelink UE 115 as a sidelink sync UE (e.g., the UE 115 c). When operating as a sidelink sync UE, the UE 115 may broadcast S-SSBs which may include synchronization signals (e.g., PSS and/or SSS) and sidelink system information, such as a sidelink BWP configuration, one or more sidelink transmit resource pools, and/or one or more sidelink receive resource pools, S-SSB transmission related parameters (e.g., sidelink slots 238 configured for S-SSB transmission and/or S-SSB transmission periodicity), and/or any other configuration information related to sidelink communications as will be discussed more fully below. Accordingly, other UEs (e.g., the UEs 115 d and 115 e) that are nearby the UE 115 c, but may be out of the coverage of the BS 105 may listen to and synchronize to the S-SSBs and communicate with each other based on the S-SSBs.

In some aspects, the UE 115 c may receive a tracking reference signal (TRS) from the UE 115 a. The UE 115 c may synchronize time and/or frequency with the UE 115 a based on the TRS. The UE 115 c may receive a physical sidelink shared channel (PSSCH) communication from the UE 115 a based on the time and/or frequency synchronization with the UE 115 a. Aspects of the present disclosure may increase time and/or frequency synchronization between the UE 115 c and the UE 115 a (e.g., sidelink UEs) thereby increasing the probability of successful decoding of communications by the UE 115 c.

FIG. 3 illustrates tracking reference signal (TRS) 322 resources according to some aspects of the present disclosure. In FIG. 3 , the x-axis represents time in some arbitrary units and the y-axis represents frequency in some arbitrary units. In some aspects, a first sidelink UE (e.g., the UE 115 or the UE 800) may receive a TRS 322 from a second sidelink UE (e.g., the UE 115 or the UE 800). The first sidelink UE may receive the TRS 322 in time/frequency resources of a slot 312. The first sidelink UE may receive the TRS 322 in a slot 312 comprising one or more demodulation reference signals (DMRSs) 320. The first sidelink UE may receive the TRS 322 in a symbol 311 of the slot 312 after one or more symbols 311 that include a physical sidelink control channel (PSCCH) 318. The first sidelink UE may receive the TRS 322 in a symbol 311 of the slot 312 after the PSCCH 318 to avoid puncturing the SCI-1 carried by the PSCCH 318. The first sidelink UE may receive one or more TRSs 322 in the slot 312. The one or more TRSs 322 may be separated by TRS resource separation 310. For example, the first sidelink UE may receive multiple TRSs 322 where each TRS 322 is separated by n number of symbols 311 in a comb-n pattern. The multiple TRSs 322 may be separated by one, two, three, four, or more symbols 311 in a comb-n pattern (e.g., where n equals one, two, three, four, or more). For example, the first sidelink UE may receive TRSs 322 in symbols 311(5) and 311(9) in a comb-4 pattern.

In some aspects, the first sidelink UE may receive PSSCHs 316 in symbols 311 between the symbols 311 in which the first sidelink UE receives the TRSs 322. For example, the first sidelink UE may receive the multiple TRSs 322 in symbols 311(5) and 311(9) and receive the PSSCHs 316 in symbols 311(6)-311(8). In some aspects, the first sidelink UE may receive an indication from the second sidelink UE indicating the symbols 311 (e.g., the symbol indexes 0-13) in which the TRSs 322 will be received. In this regard, the first sidelink UE may receive the indication in a time domain resource allocation (TDRA). The TDRA may be carried by SCI-1 via the PSCCH.

In some aspects, the first sidelink UE may receive multiple TRSs 322 in a slot 312 where each TRS 322 in the slot 312 is separated by m number of frequency subchannels in a comb-m pattern. The multiple TRSs 322 may be separated by one, two, three, four, or more subchannels in a comb-m pattern (e.g., where m equals one, two, three, four, or more). For example, as shown in FIG. 3 , the TRSs 322 are transmitted in a comb-4 pattern including subchannels 313(0), 313(4), and 313(8). In some aspects, the first sidelink UE may receive an indication from the second sidelink UE indicating the subchannels in which the TRSs 322 will be transmitted. In this regard, the first sidelink UE may receive the indication in a frequency domain resource allocation (FDRA). The FDRA may be carried by SCI-1 via a PSCCH.

In some aspects, the first sidelink UE may receive the TRS 322 from the second sidelink UE in a slot 312 that includes a PSSCH 316. The slot 312 may include an indicator (e.g., a trigger) in the SCI-1 and/or the SCI-2 that indicates to the first sidelink UE that the slot 312 includes the TRS 322 and the PSSCH 316. Before decoding the SCI-2, the first sidelink UE may not know that the slot 312 includes a TRS 322. The SCI-2 may have a common configuration among the sidelink UEs, including the first sidelink UE, such that the sidelink UEs may decode the SCI-2 from each sidelink UE that is transmitting a TRS 322. The TRSs 322 may be transmitted in symbols 311 after the symbols 311 that carry the SCI-2 and the DMRS 320. The first sidelink UE may decode the SCI-2 to determine if the PSSCH 316 is intended for the first sidelink UE by matching the UE destination ID in the SCI-2 with the ID associated with the first sidelink UE.

In some aspects, the first sidelink UE may receive the TRS 322 from the second sidelink UE in a slot 312 that includes a PSSCH 316. However, the first sidelink UE may refrain from decoding the PSSCH 316. The first sidelink UE may refrain from decoding the PSSCH 316 in order to conserve the CPU resources and/or the battery power required to decode the PSSCH. In this case, the first sidelink UE may receive the TRS 322, synchronize time and/or frequency with the second sidelink UE based on the TRS 322, and ignore the PSSCH 316. The first sidelink UE may ignore the PSSCH 316 based on the PSSCH 316 being intended for another sidelink UE (e.g., a third sidelink UE). In some aspects, the first sidelink UE may choose to decode the PSSCH 316 and expend the CPU resources and battery power required to perform the PSSCH 316 decoding. The first sidelink UE may determine that the PSSCH 316 is not intended for the first sidelink UE based on the sidelink UE destination identifier in the SCI-2 identifying a sidelink UE other than the first sidelink UE. The first sidelink UE may determine the UE transmitter of the TRS 322 by decoding the SCI-1. The SCI-1 may include a UE source ID that identifies the second sidelink UE as the transmitter of the TRS 322. In this manner, the first sidelink UE may receive TRSs 322 from multiple sidelink UEs on a frequent basis increasing the accuracy of the time/frequency synchronization with each TRS 322 transmitter. The first sidelink UE may synchronize time and/or frequency with each of the sidelink UEs that the first sidelink UE receives TRSs 322 from. In some aspects, the first sidelink UE may synchronize time and/or frequency with a subset of the sidelink UEs that the first sidelink UE receives TRSs 322 from. The first sidelink UE may update time/frequency tracking loops for each of the sidelink UEs and/or a subset of the sidelink UEs that it receives TRSs 322 from.

In some aspects, the first sidelink UE may receive a communication in a slot 312 including a TRS 322 and a PSSCH 316 from the second sidelink UE. The first sidelink UE may utilize the TRSs 322 received in the slot 312 to synchronize time and/or frequency with the second sidelink UE before decoding the PSSCH 316. In this manner, the time and/or frequency may be better synchronized with the second sidelink UE increasing the probability of successful decoding of the PSSCH 316 as compared to decoding the PSSCH 316 before synchronizing the time and/or frequency with the second sidelink UE.

FIG. 4 illustrates tracking reference signal (TRS) 322 resources according to some aspects of the present disclosure. In FIG. 4 , the x-axis represents time in some arbitrary units and the y-axis represents frequency in some arbitrary units. In some aspects, the first sidelink UE may receive the TRS 322 (e.g., a standalone TRS 322) from the second sidelink UE in a slot 312 that does not include a PSSCH as shown in FIG. 4 . In this regard, the first sidelink UE may receive a communication from the second sidelink UE that includes an AGC symbol 314, one or more TRSs 322, and a guard symbol 324. The slot 312 may or may not include a PSCCH 318. In some aspects, the TRSs 322 may be transmitted in adjacent symbols 311. The adjacent symbols 311 may be contiguous with no gaps between the symbols 311. As shown in FIG. 4 , the TRSs 322 may be transmitted in contiguous symbols 311(1)-311(8). Any number of TRSs 322 may be transmitted within any symbol 311. Any number of symbols 311 may include TRSs 322. Additionally or alternatively, the first symbol 311(0) may carry the AGC, the next 3 symbols 311(1)-311(3) may or may not carry a PSCCH 318, and the symbols 311(4)-311(8) following the PSCCH 318 may carry the TRSs 322. In the case where the slot 312 does not include the PSCCH 318, the TRSs 322 do not avoid the symbols where the PSCCH 318 may be carried (e.g., symbols 311(1)-311(3)) and the TRSs 322 may be transmitted in symbols beginning after the AGC symbol 314 (e.g., symbols 311(1)-311(8)). In the case where the slot 312 includes the PSCCH 318 (e.g., PSCCH 318 in symbols 311(1)-311(3)), the first TRS 322 may begin after the last PSCCH 312 symbol. For example, if the last PSCCH 318 symbol is symbol 311(3), the first TRS 322 may be in symbol 311(4). The PSCCH 318 may carry an SCI 420 (e.g., SCI-1 and/or SCI-2). The first sidelink UE may decode the SCI 420. The SCI 420 (e.g., the SCI-1 and/or the SCI-2) may indicate (e.g., trigger) to the first sidelink UE that the symbols 311 after the PSCCH 318 may carry the TRSs 322. The first sidelink UE may receive multiple TRSs 322 after the PSCCH 318 where each TRS 322 in the slot 312 is separated by m number of frequency subchannels 313 in a comb-m pattern. The multiple TRSs 322 may be separated by one, two, three, four, or more subchannels 313 in a comb-m pattern (e.g., where m equals one, two, three, four, or more). For example, in symbol 311(5), a TRS 322 may be transmitted in subchannel 313(0), a TRS may be transmitted in subchannel 313(4), a TRS may be transmitted in subchannel 313(8), etc. In the next symbol 311(6), the TRS 322 may be shifted by one subchannel, a TRS 322 may be transmitted in subchannels 313(1), 313(5), and 313(9). The shifting of the subchannels in subsequent symbols 311 may continue for a number of symbols 311 (e.g., 4 symbols) as shown in FIG. 4 .

Additionally or alternatively, each of the TRSs 322 may be transmitted in the same frequency subchannel(s) 313 without shifting the subchannel in subsequent slots 312. The first sidelink UE may receive a configuration from the second sidelink UE indicating the time/frequency resources associated with the TRSs 322. In some aspects, the first sidelink ULE may receive an indication in a TDRA from the second sidelink UE indicating the symbols 311 in which the TRSs 322 will be transmitted. The TDRA may be carried by SCI-1 via the PSCCH 318. In some aspects, the first sidelink UE may receive an indication in a FDRA from the second sidelink UE indicating the subchannels 313 in which the TRSs 322 will be transmitted. The FDRA may be carried by SCI-1 via the PSCCH 318.

In some aspects, the first sidelink UE may receive an indicator (e.g., a trigger) indicating the second sidelink UE will transmit a TRS 322 to the first sidelink UE. The first sidelink UE may receive the trigger from the second sidelink UE via SCI 420 (e.g., SCI-1 or SCI-2). The first sidelink UE may receive the TRS 322 aperiodically based on the trigger. In some aspects, the SCI 420 trigger indicating a TRS 322 will be transmitted to the first sidelink UE may be transmitted in the same slot 312 as the TRS 322. Additionally or alternatively, as shown by the arrow in FIG. 4 , the SCI 420 trigger may be transmitted in a slot prior to the slot 312 in which the TRS 322 will be transmitted. The slot prior to the slot 312 in which the TRS 322 will be transmitted may be one or more slots prior to the slot 312. The SCI-1 and/or the SCI-2 may indicate an index associated with the slot 312 that includes the TRS 322.

In some aspects the first sidelink UE may receive a communication including a TRS 322 from the second sidelink UE in a slot that does not include a PSSCH before (e.g., immediately before) receiving a communication from the second sidelink UE that does include a PSSCH. The first sidelink UE may receive the TRS 322 in a slot 312 without a PSSCH as shown in FIG. 4 , before receiving a communication in a slot 312 with a PSSCH 316 as shown in FIG. 3 . The first sidelink UE may receive the TRS before the PSSCH in order to synchronize time and/or frequency (e.g., perform time/frequency compensation) with the second sidelink UE before decoding the PSSCH 316. In this manner, the time and/or frequency may be better synchronized with the second sidelink UE increasing the probability of successful decoding of the PSSCH 316 as compared to receiving the TRS 322 in a slot after the PSSCH 316.

FIG. 5 illustrates a wireless communication network 500 according to some aspects of the present disclosure. The wireless communications network 500 may include sidelink UEs 115 a, 115 b, 115 c, 115 f, 115 i, 115 j, and 115 k which may be examples of a UE 115 as described with reference to FIG. 1 or FIG. 2 . The UE 115 a may communicate via sidelink communication to the UEs 115 b, 115 c, 115 f, 115 i, 115 j, and 115 k.

In some aspects, the UE 115 c may receive a TRS from the UE 115 a that is intended for a plurality of sidelink UEs. The plurality of sidelink UEs may include the UE 115 c. In this regard, the UE 115 a may transmit the TRS in a groupcast communication. The groupcast communication may be intended for a group 510 of sidelink UEs to receive the TRS and synchronize time and/or frequency to the UE 115 a. The UE 115 a may conserve resources by transmitting the TRS in a groupcast communication as compared to transmitting the TRSs in unicast communications to each member of the group 510. The groupcast communication may include a group ID that identifies the group 510 of sidelink UEs to receive the TRS. In this regard, the UE 115 c may receive (e.g., be assigned) a group ID associated with group 510(1) via SCI-2 (e.g., a codepoint in the SCI-2) in the groupcast communication or a unicast communication from the UE 115 a. Additionally or alternatively, the UE 115 c may receive (e.g., be assigned) a group ID associated with group 510(1) from a BS (e.g., the BS 105 or 900) via RRC signaling. The UE 115 c may determine that the groupcast ID associated with group 510(1) includes the UE 115 c's ID. Based on the UE 115 c determining that the groupcast ID associated with group 510(1) includes the UE 115 c's ID, the UE 115 c may receive the TRS from the UE 115 a. The UE 115 c may aperiodically receive the TRS based on whether the SCI-2 indicates a groupcast ID that includes the UE 115 c's ID. The UE 115 c may update time/frequency tracking loop with the UE 115 a based on the TRS carried by the groupcast communication. The UE 115 c may receive a TRS from multiple sidelink UEs and keep separate time/frequency tracking loops for each of the multiple sidelink UEs. For example, the UE 115 c may receive a TRS from the UE 115 b and/or the UE 115 f in addition to the UE 115 a.

In some aspects, the UE 115 c may periodically receive the TRS from the UE 115 a. In this regard, the ULE 115 c may receive an indicator of the TRS periodicity from the UE 115 a via a PSSCH, a PSCCH, SCI, an RRC message, a MAC-CE message, or other suitable communication. Additionally or alternatively, the UE 115 c may receive an indicator of the TRS periodicity from a BS (e.g., the BS 105 or 900) via a PDCCH, a PDSCH, DCI, an RRC message, a MAC-CE message, or other suitable communication.

The TRS periodicity may be based on a mobility associated with the UE 115 c and/or the UE 115 a. For example, the UE 115 c and/or the UE 115 a may be a stationary device (e.g., an IoT device such as a sensor) configured with a longer TRS periodicity as compared to a mobile UE (e.g., a vehicle or a smartphone) configured with a shorter TRS periodicity. A higher mobility device may be configured with a shorter TRS period in order to update the time/frequency synchronization at a higher frequency to compensate for a Doppler frequency shift. In some aspects, the sidelink UEs may receive TRSs at different periodicities based on the sidelink UE's mobility. For example, the UE 115 a (e.g., the TRS TXer) may form groups 510 of sidelink UEs (e.g., the TRS RXers) based on the TRS RXers mobility. For example, the UE 115 a (e.g., the TRS TXer) may form group 510(1) consisting of sidelink UEs 115 b (e.g., a smartphone), 115 c (e.g., a smartphone), and 115 f (e.g., an IoT device such as a sensor) having a low mobility. The UE 115 a (e.g., the TRS TXer) may form group 510(2) consisting of sidelink UEs 115 i (e.g., a vehicle), 115 j (e.g., a vehicle), and 115 k (e.g., a vehicle) having a high mobility. The UE 115 a may form any number of groups 510 based on the TRS RXers mobility. The UE 115 a may assign a groupcast ID to each of the groups 510(1) and 510(2). Each of the groups 510(1) and 510(2) may have a different TRS periodicity. The UE 115 a may groupcast the TRSs based on the periodicity assigned to each group 510(1) and 510(2).

FIG. 6A is a signaling diagram of a communication method 600 according to some aspects of the present disclosure. Actions of the communication method 600 can be executed by a computing device (e.g., a processor, processing circuit, and/or other suitable component) of a communication device or other suitable means for performing the actions. For example, a wireless communication device, such as the UE 115 or UE 800, may utilize one or more components, such as the processor 802, the memory 804, the tracking reference signal module 808, the transceiver 810, the modem 812, and the one or more antennas 816, to execute the aspects of method 600.

At action 602, the UE 115 b may transmit a TRS to the UE 115 c. The UE 115 b may transmit a TRS to the UE 115 c as described above with reference to FIG. 3 . The UE 115 c may receive the TRS in time/frequency resources of a slot. In some aspects, the UE 115 c may receive the TRS in a slot comprising one or more demodulation reference signals (DMRSs). In some aspects, the UE 115 c may receive the TRS in a symbol of the slot after one or more symbols that carry a physical sidelink control channel (PSCCH). The UE 115 c may receive the TRS in a symbol of the slot after the PSCCH to avoid puncturing the SCI-1 carried by the PSCCH. The UE 115 c may receive one or more TRSs in the slot. For example, the UE 115 c may receive multiple TRSs where each TRS is separated by n number of symbols in a comb-n pattern. The multiple TRSs may be separated by one, two, three, four, or more symbols in a comb-n pattern (e.g., where n equals one, two, three, four, or more). The UE 115 c may receive the multiple TRSs in symbols after the PSCCH. In some aspects, the UE 115 c may receive PSSCHs in symbols between the symbols in which the UE 115 c receives the TRSs. For example, the UE 115 c may receive the multiple TRSs in symbol indexes 5 and 9 and receive the PSSCHs in symbol indexes 6-8. In some aspects, the UE 115 c may receive an indication from the UE 115 b indicating the symbols (e.g., the symbol indexes) in which the TRSs will be received. In this regard, the UE 115 c may receive the indication in a time domain resource allocation (TDRA). The TDRA may be carried by SCI-1 via the PSCCH.

In some aspects, the UE 115 c may receive multiple TRSs in a slot where each TRS in the slot is separated by m number of frequency subchannels in a comb-m pattern. The multiple TRSs may be separated by one, two, three, four, or more subchannels in a comb-m pattern (e.g., where m equals one, two, three, four, or more). In some aspects, the UE 115 c may receive an indication from the UE 115 b indicating the subchannels in which the TRSs will be transmitted. In this regard, the UE 115 c may receive the indication in a frequency domain resource allocation (FDRA). The FDRA may be carried by SCI-1 via a PSCCH.

At action 604, the UE 115 c may synchronize time and/or frequency with the UE 115 b based on the TRS received at action 602. The TRS may assist the UE 115 c to synchronize time and/or frequency tracking with the UE 115 b and each of the sidelink UEs that the UE 115 c receives TRSs from. The TRS may be a specific configuration of the CSI-RS. In some aspects, the TRS may be configured as a non-zero power (NZP) CSI-RS resource set. The TRS may allow the UE 115 c to track frequency and time variations with a high resolution (e.g., enable fine tuning of time/frequency tracking). Improved time/frequency synchronization may benefit the performance of data transfer between the UE 115 c and the UE 115 b.

At action 606, the UE 115 b may transmit a PSSCH to the UE 115 c. In this regard, the UE 115 b may transmit a communication in a slot including a TRS and a PSSCH to the UE 115 c.

At action 608, the UE 115 c may decode the PSSCH. The UE 115 c may utilize the TRSs received in the slot to synchronize time and/or frequency with the UE 115 b before decoding the PSSCH. In this manner, the time and/or frequency may be better synchronized between the UE 115 b and the UE 115 c increasing the probability of successful decoding of the PSSCH by the UE 115 c as compared to decoding the PSSCH before synchronizing the time and/or frequency.

In some aspects, the UE 115 c may receive the TRS from the UE 115 b in a slot that includes a PSSCH. However, the UE 115 c may refrain from decoding the PSSCH. In this case, the UE 115 c may receive the TRS, synchronize time and/or frequency with the UE 115 b based on the TRS, and ignore the PSSCH. The UE 115 c may ignore the PSSCH based on the PSSCH being intended for another sidelink UE (e.g., a third sidelink UE). The UE 115 c may determine that the PSSCH is not intended for the UE 115 c based on the sidelink UE destination identifier in the SCI-2 identifying a sidelink UE other than the UE 115 c.

At action 610, the UE 115 a may transmit a TRS to the UE 115 c. The UE 115 a may transmit a TRS to the UE 115 c as described above with reference to FIG. 3 . The UE 115 c may receive the TRS in time/frequency resources of a slot. In some aspects, the UE 115 c may receive the TRS in a slot comprising one or more demodulation reference signals (DMRSs). In some aspects, the UE 115 c may receive the TRS in a symbol of the slot after one or more symbols that carry a physical sidelink control channel (PSCCH). The UE 115 c may receive the TRS in a symbol of the slot after the PSCCH to avoid puncturing the SCI-1 carried by the PSCCH. The UE 115 c may receive one or more TRSs in the slot. For example, the UE 115 c may receive multiple TRSs where each TRS is separated by n number of symbols in a comb-n pattern. The multiple TRSs may be separated by one, two, three, four, or more symbols in a comb-n pattern (e.g., where n equals one, two, three, four, or more). The UE 115 c may receive the multiple TRSs in symbols after the PSCCH. In some aspects, the UE 115 c may receive PSSCHs in symbols between the symbols in which the UE 115 c receives the TRSs. For example, the UE 115 c may receive the multiple TRSs in symbol indexes 5 and 9 and receive the PSSCHs in symbol indexes 6-8. In some aspects, the UE 115 c may receive an indication from the UE 115 a indicating the symbols (e.g., the symbol indexes) in which the TRSs will be received. In this regard, the UE 115 c may receive the indication in a time domain resource allocation (TDRA). The TDRA may be carried by SCI-1 via the PSCCH.

In some aspects, the UE 115 c may receive multiple TRSs in a slot where each TRS in the slot is separated by m number of frequency subchannels in a comb-m pattern. The multiple TRSs may be separated by one, two, three, four, or more subchannels in a comb-m pattern (e.g., where m equals one, two, three, four, or more). In some aspects, the UE 115 c may receive an indication from the UE 115 a indicating the subchannels in which the TRSs will be transmitted. In this regard, the UE 115 c may receive the indication in a frequency domain resource allocation (FDRA). The FDRA may be carried by SCI-1 via a PSCCH.

At action 612, the UE 115 c may synchronize time and/or frequency with the UE 115 a based on the TRS received at action 610. The TRS may assist the UE 115 c to synchronize time and/or frequency tracking with the UE 115 a and each of the sidelink UEs that the UE 115 c receives TRSs from. The TRS may be a specific configuration of the CSI-RS. In some aspects, the TRS may be configured as a non-zero power (NZP) CSI-RS resource set. The TRS may allow the UE 115 c to track frequency and time variations with a high resolution (e.g., enable fine tuning of time/frequency tracking). Improved time/frequency synchronization may benefit the performance of data transfer between the UE 115 c and the UE 115 a.

At action 614, the UE 115 a may transmit a PSSCH to the UE 115 c. In this regard, the UE 115 a may transmit a communication in a slot including a TRS and a PSSCH to the UE 115 c.

At action 616, the UE 115 c may decode the PSSCH. The UE 115 c may utilize the TRSs received in the slot to synchronize time and/or frequency with the UE 115 a before decoding the PSSCH. In this manner, the time and/or frequency may be better synchronized between the UE 115 a and the UE 115 c increasing the probability of successful decoding of the PSSCH by the UE 115 c as compared to decoding the PSSCH before synchronizing the time and/or frequency.

In some aspects, the UE 115 c may receive the TRS from the UE 115 a in a slot that includes a PSSCH. However, the UE 115 c may refrain from decoding the PSSCH. In this case, the UE 115 c may receive the TRS, synchronize time and/or frequency with the UE 115 a based on the TRS, and ignore the PSSCH. The UE 115 c may ignore the PSSCH based on the PSSCH being intended for another sidelink. The UE 115 c may determine that the PSSCH is not intended for the UE 115 c based on the sidelink UE destination identifier in the SCI-2 identifying a sidelink UE other than the UE 115 c.

FIG. 6B is a signaling diagram of a communication method 601 according to some aspects of the present disclosure. Actions of the communication method 601 can be executed by a computing device (e.g., a processor, processing circuit, and/or other suitable component) of a communication device or other suitable means for performing the actions. For example, a wireless communication device, such as the UE 115 or UE 800, may utilize one or more components, such as the processor 802, the memory 804, the tracking reference signal module 808, the transceiver 810, the modem 812, and the one or more antennas 816, to execute the aspects of communication method 601.

At action 620, the UE 115 a may transmit a group ID to the UE 115 b. In this regard, the UE 115 a may transmit (e.g., assign) the group ID via SCI-2 (e.g., a codepoint in the SCI-2) to the UE 115 b.

At action 621, the UE 115 a may transmit a group ID to the UE 115 c. In this regard, the UE 115 a may transmit (e.g., assign) the group ID via SCI-2 (e.g., a codepoint in the SCI-2) to the UE 115 c.

At action 622, the UE 115 a may transmit a groupcast communication including a TRS. The UE 115 a may transmit a TRS that is intended for a group of sidelink UEs. The group of sidelink UEs may include the UE 115 b and the UE 115 c. The groupcast communication may be intended for a group of sidelink UEs including the UE 115 b and the UE 115 c to receive the TRS and synchronize time and/or frequency to the UE 115 a. The UE 115 a may conserve resources by transmitting the TRS in a groupcast communication as compared to transmitting the TRSs in separate unicast communications to the UE 115 b and the UE 115 c. The groupcast communication may include a group ID transmitted at actions 620 and 621 that indicate the UE 115 b and the UE 115 c to receive the TRS.

At action 624, the UE 115 b may synchronize time and/or frequency to the UE 115 a based on the TRS received at action 622. The UE 115 b may determine that the groupcast ID includes the UE 115 b's ID. Based on the UE 115 b determining that the groupcast ID includes the UE 115 b's ID, the UE 115 b may synchronize time and/or frequency to the UE 115 a based on the TRS. The UE 115 b may aperiodically receive the TRS based on whether the SCI-2 indicates a groupcast ID that includes the UE 115 b's ID.

At action 626, the UE 115 c may synchronize time and/or frequency to the UE 115 a based on the TRS received at action 622. The UE 115 c may determine that the groupcast ID includes the UE 115 c's ID. Based on the UE 115 c determining that the groupcast ID includes the UE 115 c's ID, the UE 115 c may synchronize time and/or frequency to the UE 115 a based on the TRS. The UE 115 c may aperiodically receive the TRS based on whether the SCI-2 indicates a groupcast ID that includes the UE 115 c's ID.

At action 628, the UE 115 a may transmit a PSSCH to the UE 115 c. In this regard, the UE 115 a may transmit a communication in a slot including a TRS and a PSSCH to the UE 115 c.

At action 630, the UE 115 c may decode the PSSCH. The UE 115 c may utilize the TRSs received in the slot to synchronize time and/or frequency with the UE 115 a before decoding the PSSCH. In this manner, the time and/or frequency may be better synchronized between the UE 115 a and the UE 115 c increasing the probability of successful decoding of the PSSCH by the UE 115 c as compared to decoding the PSSCH before synchronizing the time and/or frequency.

In some aspects, the UE 115 c may receive the TRS from the UE 115 a in a slot that includes a PSSCH. However, the UE 115 c may refrain from decoding the PSSCH. In this case, the UE 115 c may receive the TRS, synchronize time and/or frequency with the UE 115 a based on the TRS, and ignore the PSSCH. The UE 115 c may ignore the PSSCH based on the PSSCH being intended for another sidelink UE. The UE 115 c may determine that the PSSCH is not intended for the UE 115 c based on the sidelink UE destination identifier in the SCI-2 identifying a sidelink UE other than the UE 115 c.

At action 632, the UE 115 a may transmit a PSSCH to the UE 115 b. In this regard, the UE 115 a may transmit a communication in a slot including a TRS and a PSSCH to the UE 115 b.

At action 634, the UE 115 b may decode the PSSCH. The UE 115 b may utilize the TRSs received in the slot to synchronize time and/or frequency with the UE 115 a before decoding the PSSCH. In this manner, the time and/or frequency may be better synchronized between the UE 115 a and the UE 115 b increasing the probability of successful decoding of the PSSCH by the UE 115 b as compared to decoding the PSSCH before synchronizing the time and/or frequency.

In some aspects, the UE 115 b may receive the TRS from the UE 115 a in a slot that includes a PSSCH. However, the UE 115 b may refrain from decoding the PSSCH. In this case, the UE 115 b may receive the TRS, synchronize time and/or frequency with the UE 115 a based on the TRS, and ignore the PSSCH. The UE 115 b may ignore the PSSCH based on the PSSCH being intended for another sidelink UE. The UE 115 b may determine that the PSSCH is not intended for the UE 115 b based on the sidelink UE destination identifier in the SCI-2 identifying a sidelink UE other than the UE 115 b.

FIG. 7A is a signaling diagram of a communication method 700 according to some aspects of the present disclosure. Actions of the communication method 700 can be executed by a computing device (e.g., a processor, processing circuit, and/or other suitable component) of a communication device or other suitable means for performing the actions. For example, a wireless communication device, such as the UE 115 or UE 800, may utilize one or more components, such as the processor 802, the memory 804, the tracking reference signal module 808, the transceiver 810, the modem 812, and the one or more antennas 816, to execute the aspects of communication method 700.

At action 702, the UE 115 b may transmit an SCI (e.g., an SCI-1 or SCI-2) to the UE 115 c. In this regard, the UE 115 b may transmit the SCI via a PSCCH. The PSCCH may carry SCI-1 and/or SCI-2. The UE 115 c may decode the SCI-1 and/or the SCI-2. The SCI-1 and/or the SCI-2 may indicate (e.g., trigger) to the UE 115 c that symbols after the PSCCH may carry a TRS.

At action 704, the UE 115 b may transmit a TRS to the UE 115 c. The UE 115 b may transmit the TRS (e.g., a standalone TRS) to the UE 115 c in a slot that does not include a PSSCH. In this regard, the UE 115 b may transmit a communication to the UE 115 c that includes an AGC symbol, a PSCCH, one or more TRSs, and a guard symbol. In some aspects, the TRSs may be transmitted in adjacent symbols. The adjacent symbols may be contiguous with no gaps between the symbols. Any number of TRSs may be transmitted within any symbol. Any number of symbols may include TRSs. Additionally or alternatively, the first symbol, (e.g., symbol index 0) may carry the AGC, the next 2 or 3 symbols (e.g., symbol indexes 1 and 2 or symbol indexes 1, 2, and 3) may carry a PSCCH, and the symbols following the PSCCH (e.g., symbol indexes 4-13) may carry the TRSs.

At action 706, the UE 115 c may synchronize time and/or frequency with the UE 115 b based on the TRS received at action 704. The TRS may assist the UE 115 c to synchronize time and/or frequency tracking with the UE 115 b and each of the sidelink UEs that the UE 115 c receives TRSs from. The TRS may be a specific configuration of the CSI-RS. In some aspects, the TRS may be configured as a non-zero power (NZP) CSI-RS resource set. The TRS may allow the UE 115 c to track frequency and/or time variations with a high resolution (e.g., enable fine tuning of time/frequency tracking). Improved time/frequency synchronization may benefit the performance of data transfer between the UE 115 c and the UE 115 b.

At action 708, the UE 115 b may transmit a PSSCH to the UE 115 c. The UE 115 b may transmit the PSSCH in a slot that does not include a TRS.

At action 710, the UE 115 c may decode the PSSCH received at action 708. The UE 115 c may utilize the TRSs received at action 704 to synchronize time and/or frequency with the UE 115 b before decoding the PSSCH. In this manner, the time and/or frequency may be synchronized between the UE 115 b and the UE 115 c increasing the probability of successful decoding of the PSSCH by the UE 115 c.

At action 712, the UE 115 a may transmit an SCI (e.g., an SCI-1 or SCI-2) to the UE 115 c. In this regard, the UE 115 a may transmit the SCI via a PSCCH. The PSCCH may carry SCI-1 and/or SCI-2. The UE 115 c may decode the SCI-1 and/or the SCI-2. The SCI-1 and/or the SCI-2 may indicate (e.g., trigger) to the UE 115 c that symbols after the PSCCH may carry a TRS.

At action 714, the ULE 115 a may transmit a TRS to the UE 115 c. The UE 115 a may transmit the TRS (e.g., a standalone TRS) to the UE 115 c in a slot that does not include a PSSCH. In this regard, the UE 115 a may transmit a communication to the UE 115 c that includes an AGC symbol, a PSCCH, one or more TRSs, and a guard symbol. In some aspects, the TRSs may be transmitted in adjacent symbols. The adjacent symbols may be contiguous with no gaps between the symbols. Any number of TRSs may be transmitted within any symbol. Any number of symbols may include TRSs. Additionally or alternatively, the first symbol, (e.g., symbol index 0) may carry the AGC, the next 2 or 3 symbols (e.g., symbol indexes 1 and 2 or symbol indexes 1, 2, and 3) may carry a PSCCH, and the symbols following the PSCCH (e.g., symbol indexes 4-13) may carry the TRSs.

At action 716, the UE 115 c may synchronize time and/or frequency with the UE 115 a based on the TRS received at action 714. The TRS may assist the UE 115 c to synchronize time and/or frequency tracking with the UE 115 a and each of the sidelink UEs that the UE 115 c receives TRSs from. The TRS may be a specific configuration of the CSI-RS. In some aspects, the TRS may be configured as a non-zero power (NZP) CSI-RS resource set. The TRS may allow the UE 115 c to track frequency and time variations with a high resolution (e.g., enable fine tuning of time/frequency tracking). Improved time/frequency synchronization may benefit the performance of data transfer between the UE 115 c and the UE 115 a.

At action 718, the ULE 115 a may transmit a PSSCH to the UE 115 c. The UE 115 a may transmit the PSSCH in a slot that does not include a TRS.

At action 720, the UE 115 c may decode the PSSCH received at action 718. The UE 115 c may utilize the TRSs received at action 714 to synchronize time and/or frequency with the UE 115 a before decoding the PSSCH. In this manner, the time and/or frequency may be synchronized between the UE 115 a and the UE 115 c increasing the probability of successful decoding of the PSSCH by the UE 115 c.

FIG. 7B a signaling diagram of a communication method 701 according to some aspects of the present disclosure. Actions of the communication method 701 can be executed by a computing device (e.g., a processor, processing circuit, and/or other suitable component) of a communication device or other suitable means for performing the actions. For example, a wireless communication device, such as the UE 115 or UE 800, may utilize one or more components, such as the processor 802, the memory 804, the tracking reference signal module 808, the transceiver 810, the modem 812, and the one or more antennas 816, to execute the aspects of communication method 701.

At action 722, the UE 115 a may transmit a group ID to the UE 115 b. In this regard, the UE 115 a may transmit (e.g., assign) the group ID via SCI-2 (e.g., a codepoint in the SCI-2) to the UE 115 b.

At action 724, the UE 115 a may transmit a group ID to the UE 115 c. In this regard, the UE 115 a may transmit (e.g., assign) the group ID via SCI-2 (e.g., a codepoint in the SCI-2) to the UE 115 c.

At action 726, the UE 115 a may transmit an SCI (e.g., an SCI-1 or SCI-2) to the UE 115 c. In this regard, the UE 115 a may transmit the SCI via a PSCCH. The PSCCH may carry SCI-1 and/or SCI-2. The UE 115 c may decode the SCI-1 and/or the SCI-2. The SCI-1 and/or the SCI-2 may indicate (e.g., trigger) to the UE 115 c that symbols after the PSCCH may carry a TRS.

At action 727, the UE 115 a may transmit an SCI (e.g., an SCI-1 or SCI-2) to the UE 115 b. In this regard, the UE 115 a may transmit the SCI via a PSCCH. The PSCCH may carry SCI-1 and/or SCI-2. The UE 115 b may decode the SCI-1 and/or the SCI-2. The SCI-1 and/or the SCI-2 may indicate (e.g., trigger) to the UE 115 b that the symbols after the PSCCH may carry a TRS.

At action 728, the UE 115 a may transmit a groupcast communication including a TRS. The UE 115 a may transmit a TRS that is intended for a group of sidelink UEs. The group of sidelink UEs may include the UE 115 b and the UE 115 c. The groupcast communication may be intended for the UE 115 b and the UE 115 c to receive the TRS and synchronize time and/or frequency to the UE 115 a. The UE 115 a may conserve resources by transmitting the TRS in a groupcast communication as compared to transmitting the TRSs in separate unicast communications to the UE 115 b and the UE 115 c. The groupcast communication may include a group ID transmitted at actions 722 and 724 that identifies the UE 115 b and the UE 115 c to receive the TRS.

At action 730, the UE 115 b may synchronize time and/or frequency to the UE 115 a based on the TRS received at action 728. The UE 115 b may determine that the groupcast ID includes the UE 115 b's ID. Based on the UE 115 b determining that the groupcast ID includes the UE 115 b's ID, the UE 115 b may synchronize time and/or frequency to the UE 115 a based on the TRS. The UE 115 b may aperiodically receive the TRS based on whether the SCI-2 indicates a groupcast ID that includes the UE 115 b's ID.

At action 732, the UE 115 c may synchronize time and/or frequency to the UE 115 a based on the TRS received at action 728. The UE 115 c may determine that the groupcast ID includes the UE 115 c's ID. Based on the UE 115 c determining that the groupcast ID includes the UE 115 c's ID, the UE 115 c may synchronize time and/or frequency to the UE 115 a based on the TRS. The UE 115 c may aperiodically receive the TRS based on whether the SCI-2 indicates a groupcast ID that includes the UE 115 c's ID.

At action 734, the UE 115 a may transmit a PSSCH to the UE 115 c. The UE 115 a may transmit the PSSCH in a slot that does not include a TRS.

At action 736, the UE 115 c may decode the PSSCH received at action 734. The UE 115 c may utilize the TRSs received at action 728 to synchronize time and/or frequency with the UE 115 a before decoding the PSSCH. In this manner, the time and/or frequency may be synchronized between the UE 115 a and the UE 115 c increasing the probability of successful decoding of the PSSCH by the UE 115 c.

At action 738, the UE 115 a may transmit a PSSCH to the UE 115 b. The UE 115 a may transmit the PSSCH in a slot that does not include a TRS.

At action 740, the UE 115 b may decode the PSSCH received at action 738. The UE 115 b may utilize the TRSs received at action 728 to synchronize time and/or frequency with the UE 115 a before decoding the PSSCH. In this manner, the time and/or frequency may be synchronized between the UE 115 a and the UE 115 b increasing the probability of successful decoding of the PSSCH by the UE 115 b.

FIG. 8 is a block diagram of an exemplary UE 800 according to some aspects of the present disclosure. The UE 800 may be the UE 115 in the network 100 or 200 as discussed above. As shown, the UE 800 may include a processor 802, a memory 804, a tracking reference signal module 808, transceiver 810 including a modem subsystem 812 and a radio frequency (RF) unit 814, and one or more antennas 816. These elements may be coupled with each other and in direct or indirect communication with each other, for example via one or more buses.

The processor 802 may include a central processing unit (CPU), a digital signal processor (DSP), an application specific integrated circuit (ASIC), a controller, a field programmable gate array (FPGA) device, another hardware device, a firmware device, or any combination thereof configured to perform the operations described herein. The processor 802 may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.

The memory 804 may include a cache memory (e.g., a cache memory of the processor 802), random access memory (RAM), magnetoresistive RAM (MRAM), read-only memory (ROM), programmable read-only memory (PROM), erasable programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM), flash memory, solid state memory device, hard disk drives, other forms of volatile and non-volatile memory, or a combination of different types of memory. In some instances, the memory 804 includes a non-transitory computer-readable medium. The memory 804 may store instructions 806. The instructions 806 may include instructions that, when executed by the processor 802, cause the processor 802 to perform the operations described herein with reference to the UEs 115 in connection with aspects of the present disclosure, for example, aspects of FIGS. 2-7 and 10-11 . Instructions 806 may also be referred to as code. The terms “instructions” and “code” should be interpreted broadly to include any type of computer-readable statement(s). For example, the terms “instructions” and “code” may refer to one or more programs, routines, sub-routines, functions, procedures, etc. “Instructions” and “code” may include a single computer-readable statement or many computer-readable statements.

The tracking reference signal module 808 may be implemented via hardware, software, or combinations thereof. For example, the tracking reference signal module 808 may be implemented as a processor, circuit, and/or instructions 806 stored in the memory 804 and executed by the processor 802.

In some aspects, the tracking reference signal module 808 may be configured to receive a tracking reference signal (TRS) from a second sidelink UE. The tracking reference signal module 808 may be configured to synchronize time and/or frequency with the second sidelink UE based on the TRS. The tracking reference signal module 808 may be configured to receive a physical sidelink shared channel (PSSCH) communication based on the time and/or frequency synchronization with the second sidelink UE.

As shown, the transceiver 810 may include the modem subsystem 812 and the RF unit 814. The transceiver 810 can be configured to communicate bi-directionally with other devices, such as the BSs 105 and/or the UEs 115. The modem subsystem 812 may be configured to modulate and/or encode the data from the memory 804 and the tracking reference signal module 808 according to a modulation and coding scheme (MCS), e.g., a low-density parity check (LDPC) coding scheme, a turbo coding scheme, a convolutional coding scheme, a digital beamforming scheme, etc. The RF unit 814 may be configured to process (e.g., perform analog to digital conversion or digital to analog conversion, etc.) modulated/encoded data from the modem subsystem 812 (on outbound transmissions) or of transmissions originating from another source such as a UE 115 or a BS 105. The RF unit 814 may be further configured to perform analog beamforming in conjunction with the digital beamforming. Although shown as integrated together in transceiver 810, the modem subsystem 812 and the RF unit 814 may be separate devices that are coupled together to enable the UE 800 to communicate with other devices.

The RF unit 814 may provide the modulated and/or processed data, e.g. data packets (or, more generally, data messages that may contain one or more data packets and other information), to the antennas 816 for transmission to one or more other devices. The antennas 816 may further receive data messages transmitted from other devices. The antennas 816 may provide the received data messages for processing and/or demodulation at the transceiver 810. The antennas 816 may include multiple antennas of similar or different designs in order to sustain multiple transmission links. The RF unit 814 may configure the antennas 816.

In some instances, the UE 800 can include multiple transceivers 810 implementing different RATs (e.g., NR and LTE). In some instances, the UE 800 can include a single transceiver 810 implementing multiple RATs (e.g., NR and LTE). In some instances, the transceiver 810 can include various components, where different combinations of components can implement RATs.

In some aspects, the processor 802 may be coupled to the memory 804, the tracking reference signal module 808, and/or the transceiver 810. The processor 802 and may execute operating system (OS) code stored in the memory 804 in order to control and/or coordinate operations of the tracking reference signal module 808 and/or the transceiver 810. In some aspects, the processor 802 may be implemented as part of the tracking reference signal module 808.

FIG. 9 is a block diagram of an exemplary BS 900 according to some aspects of the present disclosure. The BS 900 may be a BS 105 as discussed above. As shown, the BS 900 may include a processor 902, a memory 904, an tracking reference signal module 908, a transceiver 910 including a modem subsystem 912 and a RF unit 914, and one or more antennas 916. These elements may be coupled with each other and in direct or indirect communication with each other, for example via one or more buses.

The processor 902 may have various features as a specific-type processor. For example, these may include a CPU, a DSP, an ASIC, a controller, a FPGA device, another hardware device, a firmware device, or any combination thereof configured to perform the operations described herein. The processor 902 may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.

The memory 904 may include a cache memory (e.g., a cache memory of the processor 902), RAM, MRAM, ROM, PROM, EPROM, EEPROM, flash memory, a solid state memory device, one or more hard disk drives, memristor-based arrays, other forms of volatile and non-volatile memory, or a combination of different types of memory. In some instances, the memory 904 may include a non-transitory computer-readable medium. The memory 904 may store instructions 906. The instructions 906 may include instructions that, when executed by the processor 902, cause the processor 902 to perform operations described herein, for example, aspects of FIGS. 2-7 and 10-11 . Instructions 906 may also be referred to as code, which may be interpreted broadly to include any type of computer-readable statement(s).

The tracking reference signal module 908 may be implemented via hardware, software, or combinations thereof. For example, the tracking reference signal module 908 may be implemented as a processor, circuit, and/or instructions 906 stored in the memory 904 and executed by the processor 902.

The tracking reference signal module 908 may be used for various aspects of the present disclosure, for example, aspects of FIGS. 2-7 and 10-11 . In some aspects, the tracking reference signal module 908 may be configured to transmit (e.g., assign) a group ID to a UE (e.g., the UE 115 or 800) via RRC signaling. In some aspects, the tracking reference signal module 908 may be configured to transmit a TRS periodicity to a UE (e.g., the UE 115 or 800) via RRC signaling.

Additionally or alternatively, the tracking reference signal module 908 can be implemented in any combination of hardware and software, and may, in some implementations, involve, for example, processor 902, memory 904, instructions 906, transceiver 910, and/or modem 912.

As shown, the transceiver 910 may include the modem subsystem 912 and the RF unit 914. The transceiver 910 can be configured to communicate bi-directionally with other devices, such as the UEs 115 and/or 600. The modem subsystem 912 may be configured to modulate and/or encode data according to a MCS, e.g., a LDPC coding scheme, a turbo coding scheme, a convolutional coding scheme, a digital beamforming scheme, etc. The RF unit 914 may be configured to process (e.g., perform analog to digital conversion or digital to analog conversion, etc.) modulated/encoded data from the modem subsystem 912 (on outbound transmissions) or of transmissions originating from another source such as a UE 115 or UE 800. The RF unit 914 may be further configured to perform analog beamforming in conjunction with the digital beamforming. Although shown as integrated together in transceiver 910, the modem subsystem 912 and/or the RF unit 914 may be separate devices that are coupled together at the BS 900 to enable the BS 900 to communicate with other devices.

The RF unit 914 may provide the modulated and/or processed data, e.g. data packets (or, more generally, data messages that may contain one or more data packets and other information), to the antennas 916 for transmission to one or more other devices. This may include, for example, a configuration indicating a plurality of sub-slots within a slot according to aspects of the present disclosure. The antennas 916 may further receive data messages transmitted from other devices and provide the received data messages for processing and/or demodulation at the transceiver 910. The antennas 916 may include multiple antennas of similar or different designs in order to sustain multiple transmission links.

In some instances, the BS 900 can include multiple transceivers 910 implementing different RATs (e.g., NR and LTE). In some instances, the BS 900 can include a single transceiver 910 implementing multiple RATs (e.g., NR and LTE). In some instances, the transceiver 910 can include various components, where different combinations of components can implement RATs.

In some aspects, the processor 902 may be coupled to the memory 904, the tracking reference signal module 908, and/or the transceiver 910. The processor 902 may execute OS code stored in the memory 904 to control and/or coordinate operations of the tracking reference signal module 908, and/or the transceiver 910. In some aspects, the processor 902 may be implemented as part of the tracking reference signal module 908. In some aspects, the processor 902 is configured to transmit via the transceiver 910, to a UE, an indicator indicating a configuration of sub-slots within a slot.

FIG. 10 is a flow diagram of a communication method 1000 according to some aspects of the present disclosure. Aspects of the method 1000 can be executed by a computing device (e.g., a processor, processing circuit, and/or other suitable component) of a wireless communication device or other suitable means for performing the aspects. For example, a wireless communication device, such as the UE 115 or UE 800, may utilize one or more components, such as the processor 802, the memory 804, the tracking reference signal module 808, the transceiver 810, the modem 812, and the one or more antennas 816, to execute aspects of method 1000. The method 1000 may employ similar mechanisms as in the networks 100 and 200 and the aspects and actions described with respect to FIGS. 2-5, 6A, 6B, 7A, and 7B. As illustrated, the method 1000 includes a number of enumerated aspects, but the method 1000 may include additional aspects before, after, and in between the enumerated aspects. In some aspects, one or more of the enumerated aspects may be omitted or performed in a different order.

At action 1010, the method 1000 includes a first sidelink UE (e.g., the UE 115 or the UE 800) receiving a tracking reference signal (TRS) from a second sidelink UE (e.g., the UE 115 or the UE 800). The first sidelink UE may receive the TRS in time/frequency resources of a slot. In some aspects, the first sidelink UE may receive the TRS in a slot comprising one or more demodulation reference signals (DMRSs). In some aspects, the first sidelink UE may receive the TRS in a symbol of the slot after one or more symbols that include a physical sidelink control channel (PSCCH). The first sidelink UE may receive the TRS in a symbol of the slot after the PSCCH to avoid puncturing the SCI-1 carried by the PSCCH. The first sidelink UE may receive one or more TRSs in the slot. For example, the first sidelink UE may receive multiple TRSs where each TRS is separated by n number of symbols in a comb-n pattern. The multiple TRSs may be separated by one, two, three, four, or more symbols in a comb-n pattern (e.g., where n equals one, two, three, four, or more). The first sidelink UE may receive the multiple TRSs in symbols after the PSCCH. In some aspects, the first sidelink UE may receive PSSCHs in symbols between the symbols in which the first sidelink UE receives the TRSs. For example, the first sidelink UE may receive the multiple TRSs in symbols 5 and 9 and receive the PSSCHs in symbols 6-8. In some aspects, the first sidelink UE may receive an indication from the second sidelink UE indicating the symbols (e.g., the symbol indexes) in which the TRSs will be received. In this regard, the first sidelink UE may receive the indication in a time domain resource allocation (TDRA). The TDRA may be carried by SCI-1 via the PSCCH.

In some aspects, the first sidelink UE may receive multiple TRSs in a slot where each TRS in the slot is separated by m number of frequency subchannels in a comb-m pattern. The multiple TRSs may be separated by one, two, three, four, or more subchannels in a comb-m pattern (e.g., where m equals one, two, three, four, or more). In some aspects, the first sidelink UE may receive an indication from the second sidelink UE indicating the subchannels in which the TRSs will be transmitted. In this regard, the first sidelink UE may receive the indication in a frequency domain resource allocation (FDRA). The FDRA may be carried by SCI-1 via a PSCCH.

In some aspects, the first sidelink UE may receive an indicator (e.g., a trigger) indicating the second sidelink UE is transmitting a TRS to the first sidelink UE. The first sidelink UE may receive the trigger from the second sidelink UE via SCI (e.g., SCI-1 or SCI-2). The first sidelink UE may receive the TRS aperiodically based on the trigger.

In some aspects, the first sidelink UE may receive the TRS from the second sidelink UE in a slot that includes a PSSCH. The slot may include an indicator (e.g., a trigger) in the SCI-1 and/or the SCI-2 that indicates to the first sidelink UE that the slot includes the TRS and the PSSCH. Before decoding the SCI-2, the first sidelink UE may not know that the slot includes a TRS. The SCI-2 may have a common configuration among the sidelink UEs including the first sidelink UE such that the sidelink UEs may decode the SCI-2 from each sidelink UE that is transmitting a TRS. The TRSs may be transmitted in symbols after the symbols that include the SCI-2 and the DMRS. The first sidelink UE may decode the SCI-2 to determine if the PSSCH is intended for the first sidelink UE by matching the UE destination ID in the SCI-2 with the ID associated with the first sidelink UE.

In some aspects, the first sidelink UE may receive the TRS from the second sidelink UE in a slot that includes a PSSCH. However, the first sidelink UE may refrain from decoding the PSSCH. In this case, the first sidelink UE may receive the TRS, synchronize time and/or frequency with the second sidelink UE based on the TRS, and ignore the PSSCH. The first sidelink UE may ignore the PSSCH based on the PSSCH being intended for another sidelink UE (e.g., a third sidelink UE). The first sidelink UE may determine that the PSSCH is not intended for the first sidelink UE based on the sidelink UE destination identifier in the SCI-2 identifying a sidelink UE other than the first sidelink UE. The first sidelink UE may determine the transmitter of the TRS by decoding the SCI-1. The SCI-1 may include a source ID that identifies the second sidelink UE as the transmitter of the TRS. In this manner, the first sidelink UE may receive TRSs from multiple sidelink UEs on a frequent basis increasing the accuracy of the time/frequency synchronization with each TRS transmitter. The first sidelink UE may synchronize time and/or frequency with each of the sidelink UEs that the first sidelink UE receives TRSs from. In some aspects, the first sidelink UE may synchronize time and/or frequency with a subset of the sidelink UEs that the first sidelink UE receives TRSs from. The first sidelink UE may update time/frequency tracking loops for each of the sidelink UEs and/or a subset of the sidelink UEs that it receives TRSs from.

In some aspects, the first sidelink UE may receive a TRS that is intended for a plurality of sidelink UEs. The plurality of sidelink UEs may include the first sidelink UE. For example, the second sidelink UE may transmit the TRS in a groupcast communication. The groupcast communication may be intended for a group of sidelink UEs to receive the TRS and synchronize time and/or frequency to the second sidelink UE. The second sidelink UE may conserve resources by transmitting the TRS in a groupcast communication as compared to transmitting the TRSs in unicast communications to each member of the group. The groupcast communication may include a group ID that identifies the group of sidelink UEs to receive the TRS. In this regard, the first sidelink UE may receive (e.g., be assigned) a group ID via SCI-2 (e.g., a codepoint in the SCI-2) in the groupcast communication or a unicast communication from the second sidelink UE. Additionally or alternatively, the first sidelink UE may receive (e.g., be assigned) a group ID from a BS (e.g., the BS 105 or 900) via RRC signaling. The first sidelink UE may determine that the groupcast ID includes the first sidelink UE's ID. Based on the first sidelink UE determining that the groupcast ID includes the first sidelink UE's ID, the first sidelink UE may receive the TRS. The first sidelink UE may aperiodically receive the TRS based on whether the SCI-2 indicates a groupcast ID that includes the first sidelink UE's ID. The first sidelink UE may update time/frequency tracking loop with the second sidelink UE based on the TRS carried by the groupcast communication.

In some aspects, the first sidelink UE may receive the TRS (e.g., a standalone TRS) from the second sidelink UE in a slot that does not include a PSSCH. In this regard, the first sidelink UE may receive a communication from the second sidelink UE that includes an AGC symbol, a PSCCH, one or more TRSs, and a guard symbol. In some aspects, the TRSs may be transmitted in adjacent symbols. The adjacent symbols may be contiguous with no gaps between the symbols. Any number of TRSs may be transmitted within any symbol. Any number of symbols may include TRSs. Additionally or alternatively, the first symbol, (e.g., symbol index 0) may carry the AGC, the next 2 or 3 symbols (e.g., symbol indexes 1 and 2 or symbol indexes 1, 2, and 3) may carry a PSCCH, and the symbols following the PSCCH (e.g., symbol indexes 4-13) may carry the TRSs. The PSCCH may carry SCI-1 and/or SCI-2. The first sidelink UE may decode the SCI-1 and/or the SCI-2. The SCI-1 and/or the SCI-2 may indicate (e.g., trigger) to the first sidelink UE that the symbols after the PSCCH may carry the TRSs. The first sidelink UE may receive multiple TRSs after the PSCCH where each TRS in the slot is separated by n number of frequency subchannels in a comb-m pattern. The multiple TRSs may be separated by one, two, three, four, or more subchannels in a comb-m pattern (e.g., where m equals one, two, three, four, or more). Additionally or alternatively, each of the TRSs may be transmitted in the same frequency subchannel(s). For example, a first TRS may be transmitted in a first subchannel (e.g., subchannel index 0), a second TRS may be transmitted in a second subchannel (e.g., subchannel index 0+x), a third TRS may be transmitted in a third subchannel (e.g., subchannel index 0+2x), etc. Each of the subchannels may be separated by x number of subchannels. The first sidelink UE may receive a configuration from the second sidelink UE indicating the time/frequency resources associated with the TRSs. In some aspects, the first sidelink UE may receive an indication in a TDRA from the second sidelink UE indicating the symbols in which the TRSs will be transmitted. The TDRA may be carried by SCI-1 via the PSCCH. In some aspects, the first sidelink UE may receive an indication in a FDRA from the second sidelink UE indicating the subchannels in which the TRSs will be transmitted. The FDRA may be carried by SCI-1 via the PSCCH.

In some aspects the first sidelink UE may receive a communication including a TRS from the second sidelink UE in a slot that does not include a PSSCH before (e.g., immediately before) receiving a communication from the second sidelink UE that does include a PSSCH. The first sidelink UE may receive the TRS in a slot without a PSSCH before receiving a communication in a slot with a PSSCH in order to synchronize time and/or frequency (e.g., perform time/frequency compensation) with the second sidelink UE before decoding the PSSCH. In this manner, the time and/or frequency may be better synchronized with the second sidelink UE increasing the probability of successful decoding of the PSSCH as compared to receiving the TRS in a slot after the PSSCH.

In some aspects the first sidelink UE may receive a communication in a slot including a TRS and a PSSCH from the second sidelink UE. The first sidelink UE may utilize the TRSs received in the slot to synchronize time and/or frequency with the second sidelink UE before decoding the PSSCH. In this manner, the time and/or frequency may be better synchronized with the second sidelink UE increasing the probability of successful decoding of the PSSCH as compared to decoding the PSSCH before synchronizing the time and/or frequency with the second sidelink UE.

Additionally or alternatively, the first sidelink UE may receive the TRS (e.g., a standalone TRS) from the second sidelink UE in a slot that does not include a PSSCH or a PSCCH. The first sidelink UE may receive the TRS from the second sidelink UE in a slot that includes an AGC symbol, one or more TRSs, and a guard symbol. In some aspects, the TRSs may be transmitted in adjacent symbols that are contiguous with no gaps between the symbols. Any number of TRSs may be transmitted within any symbol. Any number of symbols may include TRSs. The first sidelink UE may receive a communication in a previous slot that carries an SCI-1 and/or SCI-2. The SCI-1 and/or the SCI-2 in the previous slot may indicate to the first sidelink UE that a subsequent slot may carry the TRSs. For example, the SCI-1 and/or the SCI-2 may indicate a number of slots after the slot carrying the SCI-1 and/or the SCI-2 that includes the TRS. The first sidelink UE may aperiodically receive the TRS based on whether the SCI-1 and/or the SCI-2 indicates a TRS is transmitted in a subsequent slot. Additionally or alternatively, the SCI-1 and/or the SCI-2 may indicate an index associated with the slot that includes the TRS. The SCI-1 may indicate a TDRA indicating the time resources associated with the slot carrying the TRS and/or an FDRA indicating the frequency resources associated with the slot carrying the TRS.

In some aspects, the first sidelink UE may periodically receive the TRS from the second sidelink UE. In this regard, the first sidelink UE may receive an indicator of the TRS periodicity from the second sidelink UE via a PSSCH, a PSCCH, SCI, an RRC message, a MAC-CE message, or other suitable communication. Additionally or alternatively, the first sidelink UE may receive an indicator of the TRS periodicity from a BS (e.g., the BS 105 or 900) via a PDCCH, a PDSCH, DCI, an RRC message, a MAC-CE message, or other suitable communication. The TRS periodicity may be based on a mobility associated with the first sidelink UE and/or the second sidelink UE. For example, the first sidelink UE and/or the second sidelink UE may be a stationary device (e.g., an IoT device such as a sensor) configured with a longer TRS period as compared to a mobile UE (e.g., a vehicle or a smartphone) configured with a shorter TRS period. A higher mobility device may be configured with a shorter TRS period in order to update the time/frequency synchronization at a higher frequency to compensate for a Doppler frequency shift. In some aspects, the second UE may receive TRSs at different periodicities based on the sidelink UE's mobility. For example, the second sidelink UE (e.g., the TRS TXer) may form groups of sidelink UEs (e.g., the TRS RXers) based on the TRS RXers mobility. The second sidelink UE may form any number of groups based on the TRS RXers mobility. The second sidelink UE may assign a groupcast ID to each of the groups. Each of the groups may have a different TRS periodicity. The second sidelink UE may groupcast the TRSs based on the periodicity assigned to each group.

At action 1020, the method 1000 includes a first sidelink UE (e.g., the UE 115 or the UE 800) synchronizing time and/or frequency with the second sidelink UE based on the TRS received at action 1010. The TRS may assist the first sidelink UE to synchronize time and/or frequency tracking with the second sidelink UE and each of the sidelink UEs that the first sidelink UE receives TRSs from. The TRS may be a specific configuration of the CSI-RS. In some aspects, the TRS may be configured as a non-zero power (NZP) CSI-RS resource set. The TRS may allow the first sidelink UE to track frequency and time variations with a high resolution (e.g., enable fine tuning of time/frequency tracking). Improved time/frequency synchronization may benefit the performance of data transfer between the first sidelink UE and the sidelink UEs transmitting the TRSs.

At action 1030, the method 1000 includes a first sidelink UE (e.g., the UE 115 or the UE 800) receiving a PSSCH communication based on the time and/or frequency synchronization with the second sidelink UE at action 1020. The first sidelink UE may synchronize time and/or frequency with the second sidelink UE based on the TRS(s) received from the second sidelink UE at action 1010. The first sidelink UE may receive the PSSCH in the same slot as the TRS(s) and/or in a slot following the slot that includes the TRS(s). The first sidelink UE may utilize the TRSs received in the slot to synchronize time and/or frequency with the second sidelink UE before decoding the PSSCH. In this manner, the time and/or frequency may be better synchronized with the second sidelink UE increasing the probability of successful decoding of the PSSCH as compared to decoding the PSSCH before synchronizing the time and/or frequency with the second sidelink UE.

FIG. 11 is a flow diagram of a communication method 1000 according to some aspects of the present disclosure. Aspects of the method 1100 can be executed by a computing device (e.g., a processor, processing circuit, and/or other suitable component) of a wireless communication device or other suitable means for performing the aspects. For example, a wireless communication device, such as the UE 115 or UE 800, may utilize one or more components, such as the processor 802, the memory 804, the tracking reference signal module 808, the transceiver 810, the modem 812, and the one or more antennas 816, to execute aspects of method 1100. The method 1100 may employ similar mechanisms as in the networks 100 and 200 and the aspects and actions described with respect to FIGS. 2-5, 6A, 6B, 7A, and 7B. As illustrated, the method 1100 includes a number of enumerated aspects, but the method 1100 may include additional aspects before, after, and in between the enumerated aspects. In some aspects, one or more of the enumerated aspects may be omitted or performed in a different order.

At action 1110, the method 1100 includes a first sidelink UE (e.g., the UE 115 or the UE 800) transmitting a tracking reference signal (TRS) to a second sidelink UE. The TRS may be configured for the second sidelink UE to synchronize time and/or frequency with the first sidelink UE based on the TRS. In some aspects, the first sidelink UE may transmit the TRS in a slot comprising one or more demodulation reference signals (DMRSs). In some aspects, the first sidelink UE may transmit the TRS in a symbol of the slot after one or more symbols that include a physical sidelink control channel (PSCCH). The first sidelink UE may transmit the TRS in a symbol of the slot after the PSCCH to avoid puncturing the SCI-1 carried by the PSCCH. The first sidelink UE may transmit one or more TRSs in the slot. For example, the first sidelink UE may transmit multiple TRSs where each TRS is separated by n number of symbols in a comb-n pattern. The multiple TRSs may be separated by one, two, three, four, or more symbols in a comb-n pattern (e.g., where n equals one, two, three, four, or more). The first sidelink UE may transmit the multiple TRSs in symbols after the PSCCH. In some aspects, the first sidelink UE may transmit PSSCHs in symbols between the symbols in which the first sidelink UE transmit the TRSs. For example, the first sidelink UE may transmit the multiple TRSs in symbols 5 and 9 and transmit the PSSCHs in symbols 6-8. In some aspects, the first sidelink UE may transmit an indication to the second sidelink UE indicating the symbols (e.g., the symbol indexes) in which the TRSs will be transmitted. In this regard, the first sidelink UE may transmit the indication in a time domain resource allocation (TDRA). The TDRA may be carried by SCI-1 via the PSCCH.

In some aspects, the first sidelink UE may transmit multiple TRSs in a slot where each TRS in the slot is separated by m number of frequency subchannels in a comb-m pattern. The multiple TRSs may be separated by one, two, three, four, or more subchannels in a comb-m pattern (e.g., where m equals one, two, three, four, or more). In some aspects, the first sidelink UE may transmit an indication to the second sidelink UE indicating the subchannels in which the TRSs will be transmitted. In this regard, the first sidelink UE may transmit the indication in a frequency domain resource allocation (FDRA). The FDRA may be carried by SCI-1 via a PSCCH.

In some aspects, the first sidelink UE may transmit an indicator (e.g., a trigger) indicating the first sidelink UE is transmitting a TRS to the second sidelink UE. The first sidelink UE may transmit the trigger to the second sidelink UE via SCI (e.g., SCI-1 or SCI-2). The second sidelink UE may receive the TRS aperiodically based on the trigger.

In some aspects, the first sidelink UE may transmit the TRS to the second sidelink UE in a slot that includes a PSSCH. The slot may include an indicator (e.g., a trigger) in the SCI-1 and/or the SCI-2 that indicates to the second sidelink UE that the slot includes the TRS and the PSSCH. Before decoding the SCI-2, the second sidelink UE may not know that the slot includes a TRS. The SCI-2 may have a common configuration among the sidelink UEs including the second sidelink UE such that the sidelink UEs may decode the SCI-2 from each sidelink UE that is transmitting a TRS. The TRSs may be transmitted in symbols after the symbols that include the SCI-2 and the DMRS. The second sidelink UE may decode the SCI-2 to determine if the PSSCH is intended for the second sidelink UE by matching the UE destination ID in the SCI-2 with the ID associated with the second sidelink UE.

In some aspects, the first sidelink UE may transmit the TRS to the second sidelink UE in a slot that includes a PSSCH. However, the second sidelink UE may refrain from decoding the PSSCH. In this case, the second sidelink UE may receive the TRS, synchronize time and/or frequency with the first sidelink UE based on the TRS, and ignore the PSSCH. The second sidelink UE may ignore the PSSCH based on the PSSCH being intended for another sidelink UE (e.g., a third sidelink UE). The second sidelink UE may determine that the PSSCH is not intended for the second sidelink UE based on the sidelink UE destination identifier in the SCI-2 identifying a sidelink UE other than the second sidelink UE. The second sidelink UE may determine the transmitter of the TRS by decoding the SCI-1. The SCI-1 may include a source ID that identifies the first sidelink UE as the transmitter of the TRS. In this manner, the second sidelink UE may receive TRSs from multiple sidelink UEs on a frequent basis increasing the accuracy of the time/frequency synchronization with each TRS transmitter. The first sidelink UE may synchronize time and/or frequency with each of the sidelink UEs that the first sidelink UE receives TRSs from. In some aspects, the second sidelink UE may synchronize time and/or frequency with a subset of the sidelink UEs that the second sidelink UE receives TRSs from. The second sidelink UE may update time/frequency tracking loops for each of the sidelink UEs and/or a subset of the sidelink UEs that it receives TRSs from.

In some aspects, the first sidelink UE may transmit a TRS that is intended for a plurality of sidelink UEs. The plurality of sidelink UEs may include the second sidelink UE. For example, the first sidelink UE may transmit the TRS in a groupcast communication. The groupcast communication may be intended for a group of sidelink UEs to receive the TRS and synchronize time and/or frequency to the first sidelink UE. The first sidelink UE may conserve resources by transmitting the TRS in a groupcast communication as compared to transmitting the TRSs in unicast communications to each member of the group. The groupcast communication may include a group ID that identifies the group of sidelink UEs to receive the TRS. In this regard, the first sidelink UE may transmit (e.g., assign) a group ID via SCI-2 (e.g., a codepoint in the SCI-2) in the groupcast communication or a unicast communication to the second sidelink UE. Additionally or alternatively, the second sidelink UE may receive (e.g., be assigned) a group ID from a BS (e.g., the BS 105 or 900) via RRC signaling. The second sidelink UE may determine that the groupcast ID includes the second sidelink UE's ID. Based on the second sidelink UE determining that the groupcast ID includes the second sidelink UE's ID, the second sidelink UE may receive the TRS. The second sidelink UE may aperiodically receive the TRS based on whether the SCI-2 indicates a groupcast ID that includes the second sidelink UE's ID. The second sidelink UE may update time/frequency tracking loop with the first sidelink UE based on the TRS carried by the groupcast communication.

In some aspects, the first sidelink UE may transmit the TRS (e.g., a standalone TRS) to the second sidelink UE in a slot that does not include a PSSCH. In this regard, the first sidelink UE may transmit a communication to the second sidelink UE that includes an AGC symbol, a PSCCH, one or more TRSs, and a guard symbol. In some aspects, the TRSs may be transmitted in adjacent symbols. The adjacent symbols may be contiguous with no gaps between the symbols. Any number of TRSs may be transmitted within any symbol. Any number of symbols may include TRSs. Additionally or alternatively, the first symbol, (e.g., symbol index 0) may carry the AGC, the next 2 or 3 symbols (e.g., symbol indexes 1 and 2 or symbol indexes 1, 2, and 3) may carry a PSCCH, and the symbols following the PSCCH (e.g., symbol indexes 4-13) may carry the TRSs. The PSCCH may carry SCI-1 and/or SCI-2. The second sidelink UE may decode the SCI-1 and/or the SCI-2. The SCI-1 and/or the SCI-2 may indicate (e.g., trigger) to the second sidelink UE that the symbols after the PSCCH may carry the TRSs. The first sidelink UE may transmit multiple TRSs after the PSCCH where each TRS in the slot is separated by n number of frequency subchannels in a comb-m pattern. The multiple TRSs may be separated by one, two, three, four, or more subchannels in a comb-m pattern (e.g., where m equals one, two, three, four, or more).

Additionally or alternatively, each of the TRSs may be transmitted in the same frequency subchannel(s). For example, a first TRS may be transmitted in a first subchannel (e.g., subchannel index 0), a second TRS may be transmitted in a second subchannel (e.g., subchannel index 0+x), a third TRS may be transmitted in a third subchannel (e.g., subchannel index 0+2x), etc. Each of the subchannels may be separated by x number of subchannels. The first sidelink UE may transmit a configuration to the second sidelink UE indicating the time/frequency resources associated with the TRSs. In some aspects, the first sidelink UE may transmit an indication in a TDRA to the second sidelink UE indicating the symbols in which the TRSs will be transmitted. The TDRA may be carried by SCI-1 via the PSCCH. In some aspects, the first sidelink UE may transmit an indication in a FDRA to the second sidelink UE indicating the subchannels in which the TRSs will be transmitted. The FDRA may be carried by SCI-1 via the PSCCH.

In some aspects the first sidelink UE may transmit a communication including a TRS to the second sidelink UE in a slot that does not include a PSSCH before (e.g., immediately before) transmitting a communication to the second sidelink UE that does include a PSSCH. The first sidelink UE may transmit the TRS in a slot without a PSSCH before transmitting a communication in a slot with a PSSCH in order to synchronize time and/or frequency (e.g., perform time/frequency compensation) with the second sidelink UE before the second sidelink UE decodes the PSSCH. In this manner, the time and/or frequency may be better synchronized between the first sidelink UE and the second sidelink UE increasing the probability of successful decoding of the PSSCH by the second sidelink UE as compared to receiving the TRS in a slot after the PSSCH.

In some aspects the first sidelink UE may transmit a communication in a slot including a TRS and a PSSCH to the second sidelink UE. The second sidelink UE may utilize the TRSs received in the slot to synchronize time and/or frequency with the first sidelink UE before decoding the PSSCH. In this manner, the time and/or frequency may be better synchronized between the first sidelink UE and the second sidelink UE increasing the probability of successful decoding of the PSSCH by the second sidelink UE as compared to decoding the PSSCH before synchronizing the time and/or frequency.

Additionally or alternatively, the first sidelink UE may transmit the TRS (e.g., a standalone TRS) to the second sidelink UE in a slot that does not include a PSSCH or a PSCCH. The first sidelink UE may transmit the TRS to the second sidelink UE in a slot that includes an AGC symbol, one or more TRSs, and a guard symbol. In some aspects, the TRSs may be transmitted in adjacent symbols that are contiguous with no gaps between the symbols. Any number of TRSs may be transmitted within any symbol. Any number of symbols may include TRSs. The first sidelink UE may transmit a communication in a previous slot that carries an SCI-1 and/or SCI-2. The SCI-1 and/or the SCI-2 in the previous slot may indicate to the second sidelink UE that a subsequent slot may carry the TRSs. For example, the SCI-1 and/or the SCI-2 may indicate a number of slots after the slot carrying the SCI-1 and/or the SCI-2 that includes the TRS. The second sidelink UE may aperiodically receive the TRS based on whether the SCI-1 and/or the SCI-2 indicates a TRS is transmitted in a subsequent slot. Additionally or alternatively, the SCI-1 and/or the SCI-2 may indicate an index associated with the slot that includes the TRS. The SCI-1 may indicate a TDRA indicating the time resources associated with the slot carrying the TRS and/or an FDRA indicating the frequency resources associated with the slot carrying the TRS.

In some aspects, the first sidelink UE may periodically transmit the TRS to the second sidelink UE. In this regard, the first sidelink UE may transmit an indicator of the TRS periodicity to the second sidelink UE via a PSSCH, a PSCCH, SCI, an RRC message, a MAC-CE message, or other suitable communication. Additionally or alternatively, the second sidelink UE may receive an indicator of the TRS periodicity from a BS (e.g., the BS 105 or 900) via a PDCCH, a PDSCH, DCI, an RRC message, a MAC-CE message, or other suitable communication. The TRS periodicity may be based on a mobility associated with the first sidelink UE and/or the second sidelink UE. For example, the first sidelink UE and/or the second sidelink UE may be a stationary device (e.g., an IoT device such as a sensor) configured with a longer TRS period as compared to a mobile UE (e.g., a vehicle or a smartphone) configured with a shorter TRS period. A higher mobility device may be configured with a shorter TRS period in order to update the time/frequency synchronization at a higher frequency to compensate for a Doppler frequency shift. In some aspects, the first UE may transmit TRSs at different periodicities based on the sidelink UE's mobility. For example, the first sidelink UE (e.g., the TRS TXer) may form groups of sidelink UEs (e.g., the TRS RXers) based on the TRS RXers mobility. The first sidelink UE may form any number of groups based on the TRS RXers mobility. The first sidelink UE may assign a groupcast ID to each of the groups. Each of the groups may have a different TRS periodicity. The first sidelink UE may groupcast the TRSs based on the periodicity assigned to each group.

At action 1120, the method 1100 includes a first sidelink UE (e.g., the UE 115 or the UE 800) transmitting a sidelink communication to the second sidelink UE synchronized in time and/or frequency with the first sidelink UE. The first sidelink UE may transmit the sidelink communication to the second sidelink UE based on the TRS transmitted at action 1110 assisting in synchronizing time and/or frequency between the first sidelink UE and the second sidelink UE. The TRS may assist the second sidelink UE to synchronize time and/or frequency tracking with the first sidelink UE and each of the sidelink UEs that the second sidelink UE receives TRSs from. The TRS may be a specific configuration of the CSI-RS. In some aspects, the TRS may be configured as a non-zero power (NZP) CSI-RS resource set. The TRS may allow the second sidelink UE to track frequency and time variations with a high resolution (e.g., enable fine tuning of time/frequency tracking). Improved time/frequency synchronization may benefit the performance of data transfer between the second sidelink UE and the sidelink UEs transmitting the TRSs.

Further aspects of the present disclosure include the following:

Aspect 1 includes a method of wireless communication performed by a first sidelink user equipment (UE), the method comprising receiving, from a second sidelink UE, a tracking reference signal (TRS); synchronizing time and/or frequency with the second sidelink UE based on the TRS; and receiving a physical sidelink shared channel (PSSCH) communication based on the time and/or frequency synchronization with the second sidelink UE.

Aspect 2 includes the method of aspect 1 further comprising receiving the TRS in a slot comprising one demodulation reference signal (DMRS), wherein the TRS is received in a symbol of the slot after a physical sidelink control channel (PSCCH) symbol.

Aspect 3 includes the method of any of aspects 1-2, wherein the receiving, from the second sidelink UE, the TRS comprises receiving the TRS in a slot comprising the PSSCH; and further comprising refraining from decoding the PSSCH.

Aspect 4 includes the method of any of aspects 1-3, further comprising receiving, from a plurality of sidelink UEs, a plurality of TRSs, wherein the plurality of sidelink UEs includes the second sidelink UE and the plurality of TRSs includes the TRS; and synchronizing time and frequency with the plurality of sidelink UEs based on the plurality of TRSs.

Aspect 5 includes the method of any of aspects 1-4, wherein the receiving the TRS comprises receiving the TRS in a communication for a third sidelink UE.

Aspect 6 includes the method of any of aspects 1-5, wherein the TRS is associated with a plurality of sidelink UEs, wherein the plurality of sidelink UEs includes the first sidelink UE.

Aspect 7 includes the method of any of aspects 1-6, further comprising receiving, from the second sidelink UE, an indicator indicating a group ID associated with the plurality of sidelink UEs.

Aspect 8 includes the method of any of aspects 1-7, wherein the receiving the TRS comprises receiving the TRS in a slot that does not comprise a PSSCH communication.

Aspect 9 includes the method of any of aspects 1-8, further comprising receiving, from the second sidelink UE via sidelink control information (SCI), a trigger indicating the TRS, wherein the receiving the TRS comprises receiving the TRS aperiodically based on the trigger.

Aspect 10 includes the method of any of aspects 1-9, wherein the receiving the TRS comprises receiving the TRS periodically based on a mobility associated with the first sidelink UE.

Aspect 11 includes a method of wireless communication performed by a first sidelink user equipment (UE), the method comprising transmitting, to a second sidelink UE, a tracking reference signal (TRS), wherein the TRS is configured for the second sidelink UE to synchronize time and frequency with the first sidelink UE based on the TRS; and transmitting, to the second sidelink UE synchronized in time and frequency with the first sidelink UE, a sidelink communication.

Aspect 12 includes the method of aspect 11, wherein the transmitting, to the second sidelink UE, the TRS comprises transmitting the TRS in a slot comprising one demodulation reference signal (DMRS), wherein the TRS is received in a symbol of the slot after a physical sidelink control channel (PSCCH) symbol.

Aspect 13 includes the method of any of aspects 11-12, wherein the transmitting, to the second sidelink UE, the TRS comprises transmitting the TRS in a slot comprising a physical sidelink shared channel (PSSCH).

Aspect 14 includes the method of any of aspects 11-13, further comprising transmitting, to a plurality of sidelink UEs, a plurality of TRSs, wherein the plurality of sidelink UEs includes the second sidelink UE and the plurality of TRSs includes the TRS; and the TRSs are configured for the plurality of sidelink UEs to synchronize time and frequency with the first sidelink UE based on the TRSs.

Aspect 15 includes the method of any of aspects 11-14, wherein the transmitting, to the second sidelink UE, the TRS comprises transmitting the TRS in a communication for a third sidelink UE.

Aspect 16 includes the method of any of aspects 11-15, wherein the transmitting, to the second sidelink UE, the TRS comprises transmitting the TRS in a slot that does not comprise a physical sidelink shared channel (PSSCH) communication.

Aspect 17 includes a non-transitory computer-readable medium storing one or more instructions for wireless communication, the one or more instructions comprising one or more instructions that, when executed by one or more processors of a first sidelink user equipment (UE), cause the one or more processors to perform any one of aspects 1-10.

Aspect 18 includes a non-transitory computer-readable medium storing one or more instructions for wireless communication, the one or more instructions comprising one or more instructions that, when executed by one or more processors of a first sidelink user equipment (UE), cause the one or more processors to perform any one of aspects 11-16.

Aspect 19 includes a first sidelink user equipment (UE) comprising one or more means to perform any one or more of aspects 1-10.

Aspect 20 includes a first sidelink user equipment (UE) comprising one or more means to perform any one or more of aspects 11-16.

Aspect 21 includes a first sidelink user equipment (UE) comprising a memory, a transceiver and at least one processor coupled to the memory and the transceiver, wherein the UE is configured to perform any one or more of aspects 1-10.

Aspect 22 includes a first sidelink user equipment (UE) comprising a memory, a transceiver and at least one processor coupled to the memory and the transceiver, wherein the BS is configured to perform any one or more of aspects 11-16.

Information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof.

The various illustrative blocks and modules described in connection with the disclosure herein may be implemented or performed with a general-purpose processor, a DSP, an ASIC, an FPGA or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices (e.g., a combination of a DSP and a microprocessor, multiple microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration).

The functions described herein may be implemented in hardware, software executed by a processor, firmware, or any combination thereof. If implemented in software executed by a processor, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium. Other examples and implementations are within the scope of the disclosure and appended claims. For example, due to the nature of software, functions described above can be implemented using software executed by a processor, hardware, firmware, hardwiring, or combinations of any of these. Features implementing functions may also be physically located at various positions, including being distributed such that portions of functions are implemented at different physical locations. Also, as used herein, including in the claims, “or” as used in a list of items (for example, a list of items prefaced by a phrase such as “at least one of” or “one or more of”) indicates an inclusive list such that, for example, a list of [at least one of A, B, or C] means A or B or C or AB or AC or BC or ABC (i.e., A and B and C).

As those of some skill in this art will by now appreciate and depending on the particular application at hand, many modifications, substitutions and variations can be made in and to the materials, apparatus, configurations and methods of use of the devices of the present disclosure without departing from the spirit and scope thereof. In light of this, the scope of the present disclosure should not be limited to that of the particular instances illustrated and described herein, as they are merely by way of some examples thereof, but rather, should be fully commensurate with that of the claims appended hereafter and their functional equivalents. 

What is claimed is:
 1. A method of wireless communication performed by a first sidelink user equipment (UE), the method comprising: receiving, from a second sidelink UE, a tracking reference signal (TRS); synchronizing time and frequency with the second sidelink UE based on the TRS; and receiving a physical sidelink shared channel (PSSCH) communication based on the time and frequency synchronization with the second sidelink UE.
 2. The method of claim 1, wherein the receiving the TRS comprises: receiving the TRS in a slot comprising one demodulation reference signal (DMRS), wherein the TRS is received in a symbol of the slot after a physical sidelink control channel (PSCCH) symbol.
 3. The method of claim 1, wherein the receiving, from the second sidelink UE, the TRS comprises: receiving the TRS in a slot comprising the PSSCH; and further comprising: refraining from decoding the PSSCH.
 4. The method of claim 1, further comprising: receiving, from a plurality of sidelink UEs, a plurality of TRSs, wherein the plurality of sidelink UEs includes the second sidelink UE and the plurality of TRSs includes the TRS; and synchronizing time and frequency with the plurality of sidelink UEs based on the plurality of TRSs.
 5. The method of claim 1, wherein the receiving the TRS comprises: receiving the TRS in a communication for a third sidelink UE.
 6. The method of claim 1, wherein the TRS is associated with a plurality of sidelink UEs, wherein the plurality of sidelink UEs includes the first sidelink UE.
 7. The method of claim 6, further comprising: receiving, from the second sidelink UE, an indicator indicating a group ID associated with the plurality of sidelink UEs.
 8. The method of claim 1, wherein the receiving the TRS comprises: receiving the TRS in a slot that does not comprise a PSSCH communication.
 9. The method of claim 1, further comprising: receiving, from the second sidelink UE via sidelink control information (SCI), a trigger indicating the TRS, wherein: the receiving the TRS comprises receiving the TRS aperiodically based on the trigger.
 10. The method of claim 1, wherein the receiving the TRS comprises: receiving the TRS periodically based on a mobility associated with the first sidelink UE.
 11. A method of wireless communication performed by a first sidelink user equipment (UE), the method comprising: transmitting, to a second sidelink UE, a tracking reference signal (TRS), wherein the TRS is configured for the second sidelink UE to synchronize time and frequency with the first sidelink UE based on the TRS; and transmitting, to the second sidelink UE synchronized in time and frequency with the first sidelink UE, a sidelink communication.
 12. The method of claim 11, wherein the transmitting, to the second sidelink UE, the TRS comprises: transmitting the TRS in a slot comprising one demodulation reference signal (DMRS), wherein the TRS is received in a symbol of the slot after a physical sidelink control channel (PSCCH) symbol.
 13. The method of claim 11, wherein the transmitting, to the second sidelink UE, the TRS comprises: transmitting the TRS in a slot comprising a physical sidelink shared channel (PSSCH).
 14. The method of claim 11, further comprising: transmitting, to a plurality of sidelink UEs, a plurality of TRSs, wherein the plurality of sidelink UEs includes the second sidelink UE and the plurality of TRSs includes the TRS; and the TRSs are configured for the plurality of sidelink UEs to synchronize time and frequency with the first sidelink UE based on the TRSs.
 15. The method of claim 11, wherein the transmitting, to the second sidelink UE, the TRS comprises: transmitting the TRS in a communication for a third sidelink UE.
 16. The method of claim 11, wherein the transmitting, to the second sidelink UE, the TRS comprises: transmitting the TRS in a slot that does not comprise a physical sidelink shared channel (PSSCH) communication.
 17. A first sidelink user equipment (UE) comprising:  a memory;  a transceiver; and  at least one processor coupled to the memory and the transceiver, wherein the first sidelink UE is configured to: receive, from a second sidelink UE, a tracking reference signal (TRS); synchronize time and frequency with the second sidelink UE based on the TRS; and receive a physical sidelink shared channel (PSSCH) communication based on the time and frequency synchronization with the second sidelink UE.
 18. The first sidelink UE of claim 17, wherein the first sidelink UE is further configured to: receive the TRS in a slot comprising one demodulation reference signal (DMRS), wherein the TRS is received in a symbol of the slot after a physical sidelink control channel (PSCCH) symbol.
 19. The first sidelink UE of claim 17, wherein the first sidelink UE is further configured to: receive the TRS in a slot comprising the PSSCH; and refrain from decoding the PSSCH.
 20. The first sidelink UE of claim 17, wherein the first sidelink UE is further configured to: receive, from a plurality of sidelink UEs, a plurality of TRSs, wherein the plurality of sidelink UEs includes the second sidelink UE and the plurality of TRSs includes the TRS; and synchronize time and frequency with the plurality of sidelink UEs based on the plurality of TRSs.
 21. The first sidelink UE of claim 17, wherein the first sidelink UE is further configured to: receive the TRS in a communication for a third sidelink UE.
 22. The first sidelink UE of claim 17, wherein the TRS is associated with a plurality of sidelink UEs, wherein the plurality of sidelink UEs includes the first sidelink UE.
 23. The first sidelink UE of claim 22, wherein the first sidelink UE is further configured to: receive, from the second sidelink UE, an indicator indicating a group ID associated with the plurality of sidelink UEs.
 24. The first sidelink UE of claim 17, wherein the first sidelink UE is further configured to: receive the TRS in a slot that does not comprise a PSSCH communication.
 25. The first sidelink UE of claim 17, wherein the first sidelink UE is further configured to: receive, from the second sidelink UE via sidelink control information (SCI), a trigger indicating the TRS; and receive the TRS aperiodically based on the trigger.
 26. The first sidelink UE of claim 17, wherein the first sidelink UE is further configured to: receive the TRS periodically based on a mobility associated with the first sidelink UE.
 27. A first sidelink user equipment (UE) comprising:  a memory;  a transceiver; and  at least one processor coupled to the memory and the transceiver, wherein the first sidelink UE is configured to: transmit, to a second sidelink UE, a tracking reference signal (TRS), wherein the TRS is configured for the second sidelink UE to synchronize time and frequency with the first sidelink UE based on the TRS; and transmit, to the second sidelink UE synchronized in time and frequency with the first sidelink UE, a sidelink communication.
 28. The first sidelink UE of claim 27, wherein the first sidelink UE is further configured to: transmit the TRS in a slot comprising a physical sidelink shared channel (PSSCH).
 29. The first sidelink UE of claim 27, wherein the first sidelink UE is further configured to: transmit, to a plurality of sidelink UEs, a plurality of TRSs, wherein the plurality of sidelink UEs includes the second sidelink UE and the plurality of TRSs includes the TRS; and the TRSs are configured for the plurality of sidelink UEs to synchronize time and frequency with the first sidelink UE based on the TRSs.
 30. The first sidelink UE of claim 27, wherein the first sidelink UE is further configured to: transmit the TRS in a slot that does not comprise a physical sidelink shared channel (PSSCH) communication. 